Section Editor: Phillip E. Blazar, MD
Bone and Joints
The spine is composed of 24 vertebrae that are organized into four regions: cervical, thoracic, lumbar, and sacral (Fig. 41).
There are seven cervical vertebrae.
Two of these are distinctly shaped: C1, the atlas (which holds up the globe of the head) and C2, the axis. Read more
Bone and Joints
The ankle joint is the articulation of the leg bones (the tibia and fibula) and the talus (the superior-most bone of the foot). This joint is formed as the tibial shaft flares out distally to form the medial malleolus. This malleolus serves as a medial buttress of the ankle and as the proximal point of origin of the deltoid ligament. The distal articular surface of the tibia (the plafond) is not perfectly flat but instead has a slight central ridge that corresponds to a central depression in the talar dome. This shape increases the congruity and stability of the joint. The fibula also flares out distally to form the lateral malleolus, the anchor point for the lateral ankle ligaments and a buttress preventing lateral displacement of the talus. The distal tibia and fibula thus form a “mortise,” an inverted U, which contains the dome of the talus. The talar dome accordingly comprises the floor of the ankle joint and articulates with the tibial plafond superiorly, the medial malleolus medially, and the lateral malleolus laterally.
The architecture of the foot can be divided into three parts: the hindfoot, the mid- foot, and the forefoot (Figs. 35 and 36).
The hindfoot consists of the talus and calcaneus and serves as the link between the ankle and the remainder of the foot. The undersurface of the talar body consists of articular cartilage that contacts the calcaneus to form the subtalar joint. Much of the talus is covered by articular cartilage with no direct muscle attachments and few areas for the entry of blood vessels into the bone. In the presence of severe fractures or dislocations, this limited vascular supply predisposes the talus to osteonecrosis. The body of the calcaneus is its largest portion and bears large compressive loads during weight bearing. The sus- tentaculum tali, a dense bony projection off the medial side of the calcaneal body, helps support the talus.
The midfoot is made up of the tarsal bones, namely, the navicular; the cuboid; and the medial, middle, and lateral cuneiforms. The midfoot meets the hindfoot at the talo- navicular and calcaneocuboid joints, which collectively are known as the transverse tarsal joint, or Chopart’s joint. The distal extent of the midfoot is the tarsometatarsal joint, or the Lisfranc joint. Medially, the navicular serves as the insertion for the posterior tibialis tendon. The cuboid, the most lateral bone, has a shallow groove on its lateral side in which the peroneus longus tendon is housed as it turns from the lateral side of the foot toward the plantar aspect.
The forefoot bones include the metatar- sals, the phalanges, and two accessory bones beneath the hallux, the medial and lateral sesamoids. The metatarsal bones link the midfoot with the toes. The great toe, or hal- lux, has two phalanges (proximal and distal) and one interphalangeal joint. The four lesser toes each have three phalanges (proximal, middle, and distal), with two intervening joints, the proximal interphalangeal (PIP) joint and the distal interphalangeal (DIP) joint. The phalanges are short tubular bones. The concave proximal phalanx base articu- lates with the metatarsal head.
The sesamoid bones beneath the metatarsophalangeal joint of the great toe have a function similar to that of the patella and increase the mechanical advantage of the muscles that flex that metatarsophalangeal joint. Because of their position at the ball of the foot, the sesamoids undergo substantial mechanical force and are often the site of overuse conditions or injuries.
An extensive array of ligaments stabilize the ankle (Fig. 37).
Medially, the deltoid ligament resists eversion (tilting up of the lateral foot) as well as rotation of the talus within the mortise. The lateral ankle ligaments resist inver- sion of the ankle joint (tilting up of the medial foot). The lateral ligament complex consists of the anterior talofibular, calcaneofibular, and posterior talofibular ligaments (Fig. 38).
The calcaneofibular ligament resists varus tilting of the joint when the ankle is in the neutral position (neither plantar flexed nor dorsiflexed). When the ankle is in the neutral position, the anterior talofibular ligament resists anterior subluxation of the talus. In plantar flexion, the anterior talofibular ligament resists inversion because in that position the ligament is oriented more vertically.
The syndesmotic ligaments (syndesmosis) stabilize the tibia to the fibula above the level of the ankle joint. This complex is composed of four major structures that resist separation (or diastasis) of the tibia from the fibula.
The subtalar joint is stabilized primarily by ligaments directly between the talus and calcaneus. The motions at the subtalar joint are inversion and eversion. The midfoot demonstrates a complex variety of motion, allowing the foot to accommodate uneven surfaces or terrain. These motions include dorsiflexion, plantar flexion, abduction, adduction, and rotation. The metatarsophalangeal joints in the forefoot are stabilized by medial and lateral collateral ligaments, a weak dorsal joint capsule, and a strong plan- tar ligament complex (the “plantar plate”). The PIP and DIP joints have a similar ligamentous structure that, along with the bony anatomy, provides stability.
Muscles of the foot and ankle are divided into two groups: intrinsic muscles, which have their bellies in the foot; and extrinsic muscles, which are based in the compartments of calf, with only their tendons cross- ing the ankle. The muscles of the superficial posterior compartment (gastrocnemius and soleus) attach to the calcaneus and are powerful plantar flexors of the ankle. The muscles of the deep posterior compartment cross the ankle behind the medial malleolus deep to the flexor retinaculum. The posterior tibialis inverts the ankle and provides dynamic support of the arch of the foot. The flexor hallucis longus and flexor digitorum longus tendons insert into the distal phalanges of the great toe and lesser toes; flex the toes; and, to a small degree, contribute to ankle plantar flexion.
The anterior compartment muscles (tibialis anterior, extensor hallucis longus, and ex- tensor digitorum longus) are extensors of the ankle. They cross the ankle anteriorly and pass deep to the extensor retinaculum to exit onto the dorsum of the foot. The tibialis anterior tendon inserts on the medial cuneiform and the base of the first metatarsal and is the primary extensor of the ankle. The extensor hallucis longus and extensor digitorum longus extend the toes.
The lateral compartment of the leg contains the peroneus longus and peroneus brevis muscles, which are important dynamic stabilizers of the lateral ankle and protect against inversion sprains. These two muscles cross the ankle behind the lateral malleolus in a shallow groove that is covered by the peroneal retinaculum. The peroneus brevis tendon inserts on the base of the fifth metatarsal, whereas the peroneus longus tendon curves around a groove in the cuboid and travels deep into the plantar aspect of the foot to insert on the base of the first metatarsal.
The intrinsic muscles are housed in the foot itself. The extensor hallucis brevis and extensor digitorum brevis muscles arise from the lateral hindfoot, and their tendons travel across the dorsum of the foot. The extensor hallucis brevis inserts on the proximal phalanx of the great toe and extends the metatarsophalangeal joint. The extensor digitorum brevis tendons extend the metatarsophalangeal joint and are innervated by a branch from the deep peroneal nerve.
On the plantar side, the plantar fascia originates from the calcaneus and inserts on the plantar plate of the metatarsophalangeal joint, which helps support the longitudinal arch of the foot. Branches of the tibial nerve innervate the plantar intrinsic foot muscles. These muscles, which abduct the toes and assist with flexion and extension, are similar in function to the intrinsic muscles of the hand.
Nerves and Blood Vessels
Five nerves supply the lower legs, ankles, and feet: the tibial, saphenous, sural, superficial peroneal, and deep peroneal nerves (Fig. 39).
The saphenous and sural nerves are purely sensory nerves, whereas the other three are mixed sensory and motor nerves. At the level of the ankle, the tibial nerve travels posterior to the medial malleolus and runs deep to the flexor retinaculum in the tarsal tunnel. It then trifurcates into the medial calcaneal, medial plantar, and lateral plantar nerve branches. High in the calf, the common peroneal nerve divides into two branches, the superficial and deep peroneal nerves. The deep peroneal nerve runs within the anterior compartment of the leg with the anterior tibial vessels and innervates the muscles there. It sends a sen- sory branch onto the dorsum of the foot with the dorsalis pedis artery, which terminates in the web space between the great toe and the second toe. The superficial peroneal nerve runs within the lateral muscle compartment of the leg, innervating the peroneus longus and brevis muscles. The nerve continues distally to supply sensation to most of the dorsum of the foot.
The ankle and foot have three major arteries providing vascular inflow: the posterior tibial, anterior tibial, and peroneal arteries (Fig. 40).
These three branches arise in the proximal leg at the “trifurcation” of the popliteal artery. (This is not a true trifurcation as the anterior tibial splits off first.) The posterior tibial artery pulse is palpable posterior to the medial malleolus. The anterior tibial artery crosses the ankle and courses on the dorsum of the foot, at which point it is renamed the dorsalis pedis artery. The peroneal artery runs within the deep posterior compartment but supplies the peroneal muscles and the lateral foot and ankle. As in the hand, these vessels have extensive anastomoses; therefore, collateral flow is often sufficient when an iso- lated arterial injury occurs.
Bones and Joints
There are four bones joined at the knee, al- though no one bone touches all three of the others. The femur from above and the tibia from below articulate to form a joint that approximates a hinge. The patella, a sesamoid bone within the quadriceps tendon, articulates with the femur to create a joint that is similar to a pulley mechanism. The tibia articulates with the second leg bone, the fibula, in a joint that allows a small degree of rotation (Figs. 24 and 25).
The ends of the femur and tibia are covered with articular cartilage, as is the surface of the patella. In between the condyles is a space called the intracondylar notch, which houses the cruciate ligaments. The femur does not articulate with the fibula. The distal ends of the femur, the condyles, can be thought of as two wheels that roll and glide along the relatively flat surface of the tibia. As the knee flexes, the femur rolls posteriorly. As the knee is straightened, the lateral plateau of the tibia reaches full extension before the medial side does; thus, at terminal extension the tibia rotates externally (with the medial plateau continuing to extend and the lateral plateau remaining motionless). These last few degrees of motion (“screw- home movement”) allow the knee to lock in full extension. As a result, the inability to fully extend the knee (a “flexion contracture”) can lead to an abnormal gait.
The function of the patella is to hold the quadriceps farther anterior to the central hinge of the knee, thus increasing the moment arm of extension. Knee extension can be considered a rotational motion around the center of the knee as seen on the lateral view. This torque can be increased by either pull- ing harder or increasing the distance between the line of pull and the axis of rotation. Increasing the distance between the line of pull of the quadriceps and the center of the knee is the goal of the patella. In fact, it is able to increase the torque by as much as 30%. The patella is consequently subject to high joint reactive forces—many times a person’s body weight, in fact.
Ligaments and Soft Tissues
The knee, unlike the hip, does not have much bony congruity to make it stable; and, unlike the shoulder, it relies less on muscles to hold it in place. Rather, it uses ligaments to secure the joint. The main ligaments are the anterior cruciate ligament (ACL), the posterior cruciate ligament (PCL), the medial collateral ligament (MCL), and the lateral collateral ligament (LCL).
The MCL is long and broad, extending from the femur to the tibial metaphysis. It prevents the loss of contact between the femur and tibia when the knee is hit from the outside. Since most people have a slight knock-knee (valgus) alignment, the MCL also prevents the joint from gapping during simple weight bearing. The LCL is much smaller (reflecting, perhaps, the reduced likelihood of being hit from the inside) and attaches to the fibula. It is the fibular attachments to the tibia, in turn, that complete the link, stabilizing the lateral side of the knee.
The cruciate ligaments are found be- tween the condyles in the intracondylar notch (Fig. 26).
The ACL resists anterior subluxation of the tibia, and the PCL resists posterior motion. The orientation of these ligaments is primarily vertical. This permits the knee to move through a wide arc of motion. In fact, you can think of the tibia as suspended from the femur by the cruciates and the hinge motion of the knee as the swinging that such suspension allows.
When the knee is flexed, the patella is stabilized by bone congruity: it sits firmly in the groove of the femoral trochlea. At full extension, there is no bony contact, and stability is achieved by the medial and lateral retinacula (ligaments that hold the patella to the femur) and the balanced tension of the muscles of the quadriceps. If the pull of the quadriceps is excessively lateral (as may be the case with a knock-knee deformity), the patella may be unstable. At the very least, this lateral pull can lead to imperfect track- ing between the articular surfaces, with greater contact on the lateral facet of the patella. This increased focal contact will in- crease pressure and, in turn, can lead to cartilage breakdown and pain.
The intra-articular space of the knee is lined with synovium, which produces lubricating and nourishing fluid. This space is bounded by a thin capsule that holds this fluid around the knee and prevents it from dissipating into the soft tissues. The extent of the pouch is at least a few inches superior to the patella, allowing a large space for injecting or aspirating the knee, if needed.
There are also two menisci (singular “meniscus”; Greek for “little moon”) in each knee, crescent-shaped cartilages that rest on the tibial plateaus (Fig. 27).
They are progressively thicker toward the periphery. Consequently, a sagittal or coronal slice (as seen in MRI, for example) looks like a wedge. The function of the meniscus is threefold: (1) it is a cushion that functions as a shock absorber; (2) it creates a greater contact area between the femur and tibia, allowing the force of weight bear- ing to be dissipated across a larger surface area; and (3) it helps stabilize the knee, much in the way a brick behind a back tire prevents a car from rolling while changing the front tire. Loss of the meniscus (as with an irreparable tear) may increase pressure in the knee along with a sense of instability.
Extension is powered by the quadriceps (innervated, primarily, by the L4 nerve root contribution to the femoral nerve) (Fig. 28).
The quadriceps, as the name implies, is composed of four muscles: three vastus muscles and the rectus femoris (Fig. 29).
The hamstrings, which power flexion, also have four parts: the two heads of the biceps, which attach laterally on the fibula; and the medial group, the semitendinosus and the semimembranosus, which attach on the tibia. The gastrocnemius, which spans the knee posteriorly, attaches to the femur and also powers knee flexion. The muscles of the knee can also help stabilize the joint, especially when the cruciates are injured: the quadriceps has a slight anterior pull and the hamstrings and gastrocnemius pull posteriorly.
Although the biceps attaches to the fibular head and the semitendinosus attaches to the anterior tibia, in terms of flexion function, the hamstrings behave as if all were attached to the posterior tibia. Understand- ing the actual attachments of the tendons becomes important when examining patients whose tendons are inflamed or injured. One tendon crossing the knee, the iliotibial band, is not easily categorized. This is the tendon of the tensor fascia lata muscle, and it receives some fibers from the gluteus maximus. The band courses down the side of the leg and attaches on the anterolateral aspect of the tibia. When the knee is flexed beyond 30°, the band passes posterior to the axis of the knee and serves as a flexor. When the knee is flexed less than 30°, however, the band passes anterior to the axis and serves as an extensor.
The muscles of the leg are housed in four distinct fascial compartments: two posterior (one deep and one superficial), one lateral, and one anterior (Fig. 30).
The superficial posterior compartment of the leg contains the gastrocnemius, soleus, and plantaris muscles, all of which are innervated by branches of the tibial nerve. The gastrocnemius muscle is composed of medial and lateral heads that arise from the posterior distal femur (Fig. 31).
Its fibers cross the knee joint (and thus flex it) and insert into the superior portion of the Achilles tendon. The soleus muscle arises from the proximal tibia and fibula and inserts more distally into the Achilles tendon. These two muscles provide the vast majority of plantar flexion strength to the ankle. The plantaris is a small vestigial muscle whose strength contribution is minimal.
The deep posterior compartment con- tains the posterior tibialis, flexor digitorum longus, and flexor hallucis longus muscles (Fig. 32).
These muscles arise from the posterior tibia, fibula, and interosseous mem- brane; are innervated by the tibial nerve; and course behind the medial malleolus. The pos- terior tibialis inserts on the navicular.The anterior compartment contains the anterior tibialis, extensor hallucis longus, ex- tensor digitorum longus, and peroneus ter- tius muscles (Fig. 33).
Innervated by the deep peroneal nerve, these muscles act as extensors of the ankle during the swing phase of gait. They arise from the anterior tibia and interosseous membrane of the leg and enter the ankle under the extensor retinaculum. Injuries to the peroneal nerve, therefore, result in a loss of active ankle dorsiflexion, a functional deficit known as a footdrop.
The lateral compartment of the leg contains the peroneus longus and peroneus brevis muscles (Fig. 34).
These muscles are innervated by the superficial peroneal nerve and act to evert the ankle and protect against inversion sprains. They arise from the fibula, and their tendons cross the ankle directly posterior to the lateral malleolus.
Nerves and Blood Vessels
Nerves and blood vessels are important components of the knee; however, for the most part, they are transient—on their way to per- form important tasks in the leg and foot. The nerves and blood vessels are located in back of the knee in the diamond-shaped popliteal fossa. The popliteal artery is an extension of the femoral artery, which changes its name as it dives posteriorly through the adductor canal. The popliteal artery gives off the genicular arteries, which supply the knee, and continues to the leg as the anterior and posterior tibial arteries. The posterior tibial artery splits and gives off the peroneal artery. The popliteal artery is at risk for damage during a knee dislocation because it is fairly well tethered within the fossa.
The nerves also pass through the popliteal fossa. In the thigh, the sciatic nerve splits into separate tibial and peroneal branches. These enter the fossa distinctly, with the peroneal laterally placed. The tibial nerve remains posterior in the calf throughout. The peroneal wraps around the fibular neck where it is subject to injury during fracture or dislocation. This nerve then splits into the deep peroneal nerve, which supplies the anterior muscles of the leg, and the superficial peroneal nerve, which supplies the muscles of the lateral compartment (the peroneus longus and brevis).
The vessels and nerves continue into the leg in distinct fascial compartments. This arrangement is somewhat counterintuitive because there are four compartments but only three nerves and arteries (the superficial posterior compartment has neither a vessel nor an artery). In addition, the lateral compartment has its own nerve (the superficial peroneal) but no blood vessel. The vessel that feeds it, the peroneal artery, actually resides in the deep posterior compartment.
Bones and Joints
The hip joint is the articulation of the femur within the acetabulum of the pelvis. The hip spans the pelvis and the proximal femur. The proximal femur contains the femoral head, which is covered with articular cartilage; the femoral neck, which connects the head to the shaft; and two bony prominences, the greater trochanter and the lesser trochanter (Fig. 16).
The greater and lesser trochanters serve as attachment points for the hip abductors and flexors, respectively. The ridge between the trochanters is called the intertrochanteric region. The intertrochanteric region and the femoral neck are the two most common sites of hip fracture in older people. Femoral neck fractures are especially troubling, not only be- cause of their lower propensity to heal but also because the blood supply to the femoral head, which courses along the neck, is prone to disruption with such fractures. Disruption of the blood supply can cause ischemia, resulting in death of the femoral head.
The femoral neck is angulated approximately 135o with respect to the shaft of the femur. In addition, the femoral neck points anteriorly approximately 15o. As a result, when a femur rests on a flat surface, the femoral head is elevated off that surface as the femoral neck inclines up out of the true anteroposterior plane. Increased anteversion causes a compensatory internal rotation of the femur (which is needed to keep the head seated in the acetabulum) and therefore ab- normal alignment of the feet or legs.
The pelvis contains a right and left hemipelvis, each of which is composed of three bones: the ilium above the hip joint, the ischium behind and below the hip joint, and the pubis medial to the hip joint. These bones unite at the center of the acetabulum. The pubis and ischium also meet to surround the obturator foramen. Congruity between the femoral head and the acetabular socket de- pends on contact as the fetus develops; accordingly, children who have developmental or congenital dislocation of the hip may have malformed femoral heads and acetabuli.
Each of the three bones of the pelvis is typically palpable. The ischial tuberosity is the bony prominence on which we sit. The iliac crest is normally felt right below the beltline laterally. The junction of the right and left pubic bones, the symphysis pubis, is palpable inferior to the umbilicus. The entire proximal femur is deeply enveloped within a sleeve of muscles; thus, with the exception of the greater trochanter laterally, it is often not directly palpable.
The hip joint is a true ball-and-socket joint and therefore inherently stable. Nonetheless, it is supplemented with ligamentous attachments. Three anterior ligaments, one from each of the pelvic bones to the femur, comprise the hip capsule. In addition, a ligamentum teres attaches from the deepest base of the acetabulum to the femoral head directly. The pelvic bones themselves are attached to the sacrum by anterior and posterior sacroiliac ligaments. The right and left sides of the pelvis are attached anteriorly at the symphysis pubis by strong ligaments.
The acetabular socket is deepened by the labrum, a lip of cartilage similar to the meniscus of the knee and the labrum of the shoulder. It is a dense fibrocartilagenous ring that surrounds the rim of the acetabulum. Because the labrum increases the depth of the acetabulum, it enhances the stability of the hip joint.
Muscles of the hip are best considered in terms of functional groups, including the flexors, the extensors, the abductors, the adductors, and the external rotators. There are no true internal rotators of the hip joint. The strongest hip flexor is the iliopsoas, a muscle formed by the fusion of the iliacus and psoas muscles (Fig. 17).
These muscles originate in the pelvis and unite to form a common tendon that inserts on the lesser trochanter. The rectus femoris, another flex- or, originates on the anterior-inferior iliac spine of the pelvis and courses down the femur, uniting with the vastus muscles to form the quadriceps tendon. The quadriceps ultimately inserts on the tibia via the patellar tendon. It thus extends the knee as well. The final flexor is the sartorius, the longest mus- cle in the body. It originates from the antero- superior iliac spine and inserts with the gra- cilis and semitendinosus on the anteromedial tibia to form the pes anserinus. Because it reaches the tibia behind the axis of the knee (in contrast to the rectus, which is located in front of the axis throughout), the sartorius is a flexor of the knee. The femoral nerve powers all of the hip flexors.
Hip extension is performed primarily by the gluteus maximus and the hamstring muscles. The gluteus maximus arises from the outer surface of the ilium and inserts on the posterior femur (Fig. 18).
It also has a common insertion with the tensor fascia lata muscle onto the iliotibial band. The hamstring muscles, namely, the biceps femoris, the semitendinosus, and the semimembranosus, all originate from the ischial tuberosity. They cross both the hip and knee joints, and insert on the tibia and fibular head (Fig. 19).
The gluteus maximus receives innervation from the inferior gluteal nerve; the hamstrings are all powered by the tibial nerve, with the exception of the short head of the biceps femoris, which receives its innervation from the peroneal part of the sciatic nerve.
The abductors of the hip joint are the gluteus medius and the gluteus minimus (Fig. 20).
Hip abduction refers to the motion of pulling the leg away from the plane of the body; but in day-to-day life, the more important function of the hip abductors is to iso- metrically resist adduction. That is, these muscles keep the pelvis horizontal when the contralateral leg is not touching the ground. The gluteus medius and minimus arise from the ilium and insert onto the greater trochanter; both are supplied by the superior gluteal nerve, The hip adductors originate from the pel- vis and insert on the femoral shaft (Fig. 21).
They are powered by the obturator nerve. This group includes the adductor longus, the adductor brevis, the adductor magnus, and the gracilis. The gracilis, unlike the other muscles in this group, inserts on the anteromedial tibia near the sartorius and semitendinosus.
The external rotators are six short muscles that course behind the joint and insert on the medial aspect of the greater trochanter. One of these, the piriformis muscle, may irritate the sciatic nerve as it exits the sciatic foramen, causing “piriformis syndrome.” The piriformis muscle also serves as a surgical landmark in the sciatic foramen above which the superior gluteal nerve and vessels can be found.
Nerves and Blood Vessels
The major nerves of the hip region all come from the lumbosacral plexus. The femoral nerve is composed of branches from L2, L3, and L4. The sciatic nerve has both a tibial branch and a peroneal branch (Fig. 22).
These travel through the hip region together but split at the popliteal fossa into distinct nerves.
The superior and inferior gluteal nerves are also branches from the posterior division of the sacral plexus.
Blood vessels enter the pelvis as branches of the aorta. The aorta branches into left and right common iliac arteries at a point located approximately at the level of the L4 vertebral body. These in turn divide into the internal and external iliac arteries at the level of the sacrum. The internal iliac provides branches to supply both the superior and inferior gluteal arteries. The external iliac crosses under the inguinal ligament and is renamed the femoral artery (Fig. 23).
The femoral artery gives off the profunda femoris and then continues on to the knee as the popliteal artery. Two other important branches of the femoral artery are the medial and lateral femoral circumflex arteries. The medial femoral circumflex artery provides the majority of blood to the femoral head. It travels up the femoral neck and is at risk for disruption with femoral neck fractures. The femoral artery pulse is readily palpable at the groin. The femoral nerve is lateral to the artery and the vein medial to it; thus, you can draw blood or obtain venous access via the groin by palpating the pulse and placing a needle or cannula medial to it.
Bone and Joints
The radius and ulna are the bones of the forearm; they connect the hand to the arm. Proximally, the main articulation is between the ulna and the distal humerus, forming the hinge of the elbow. The radius functions primarily to allow pronation and supination. At the wrist, the roles are reversed. Here, the distal radius provides the primary articulation with the carpal bones of the hand, with the ulna participating mainly in pronation and supination. The radius accepts approximately 80% of the weight transfer from the hand across the wrist. Distal to the radius and the ulna at the wrist are the carpal bones, which are organized into two rows. The proximal row consists of the scaphoid, the lunate, the triquetrum, and the pisiform. The distal row includes the trapezium, the trapezoid, the capitate, and the hamate.
The radial and the ulnar styloids are pal- pable subcutaneous landmarks at the wrist. The distal pole of the scaphoid is palpable on the palmar aspect of the wrist. The scaphoid is also palpable in the “anatomic snuff box,” the space between the extensor pollicis brevis and longus tendons to the thumb. The pisiform and the hook of the hamate are pal- pable on the volar ulnar surface of the palm.
Each finger of the hand is composed of a metacarpal bone and three phalanges, with the exception of the thumb (Fig. 13).
The thumb has a proximal and a distal phalanx articulating at a single interphalangeal joint, while the remaining fingers have proximal, middle, and distal phalanges that form a proximal interphalangeal (PIP) joint and a distal interphalangeal (DIP) joint.
Ligaments and Soft Tissues
The shaft of the radius is attached to the shaft of the ulna by an interosseous membrane. This membrane, technically a ligament, helps transfer forces from the radius (which supports the hand) to the ulna, which has the primary articulation with the humerus. Distally, the radius is attached to the proximal row of carpal bones by strong volar ligaments. The radius is attached to the ulna by the dorsal and volar radioulnar ligaments of the triangular fibrocartilage complex. The proximal and the distal rows of carpal bones are connected via a joint capsule that allows for both flexion/extension and radial/ulnar deviation of the hand. The metacarpophalangeal joints as well as the interphalangeal joints are stabilized by joint capsules and col- lateral ligaments. The ulnar collateral ligament of the thumb is especially susceptible to sprains (causing a skier’s or gamekeeper’s thumb).
There are two important transverse ligaments at the wrist. On the dorsal surface, there is an extensor retinaculum. This pulley- like structure tethers the extensor tendons close to the bones of the wrist, even as the muscle contracts. The extensor retinaculum houses six separate synovial sheaths, creating six extensor compartments. There is also a transverse covering to the volar surface, the main component of which is the trans- verse carpal ligament. This is functionally a “flexor retinaculum” and forms the roof of the carpal canal. This canal (or tunnel) houses nine flexor tendons and the median nerve as they pass into the hand from the forearm (Fig. 14).
Compression of the median nerve at the wrist causes carpal tunnel syndrome.
The muscles of the hand can be categorized into two groups: intrinsics and extrinsics. Intrinsic muscles reside exclusively in the hand itself. The extrinsic muscles have tendons that attach in the hand but their muscle bel- lies reside in the forearm. The thenar group of intrinsics powers the thumb. This group includes the abductor pollicis brevis, the op- ponens pollicis, the flexor pollicis brevis, and the adductor pollicis. The first three are sup- plied by the median nerve in the hand after it passes under the transverse carpal ligament and are responsible for opposition. A similar (but functionally less important) group of muscles, the hypothenar muscles, attach to the little finger.
The final constituents of the intrinsic group are the lumbricals and the in- terosseous muscles. The dorsal and volar (palmar) interosseous muscles originate from the metacarpals and insert on the proximal phalanges. The dorsal interosseous muscles abduct the fingers; the volar group adducts the fingers. The intrinsics also have a key role in finger flexion and extension. To un- derstand this role, the anatomy of the extrin- sics, the flexor and extensor tendons of the long fingers, must first be understood (Fig. 15)
The tendons of the finger flexor muscles pass into the wrist via the carpal tunnel. They then travel to the fingers bound by fibrous digital sheaths, an intricate pulley system that both nourishes the tendons and prevents “bowstringing” when the muscles contract.
The superficialis attaches to the middle pha- lanx, and the profundus attaches to the distal phalanx. The superficialis is, as its name im- plies, superficial; thus, to allow the profun- dus (ie, deep) flexor to reach the distal pha- lanx, it divides into two slips, and the profundus tendon passes between them (Fig 11, right). The superficialis attaches to the middle phalanx and flexes the PIP joint. The profundus, attaching to the distal phalanx, flexes the DIP joint.
Note that there is no extrinsic flexor tendon that attaches directly to the proximal phalanx. Flexion of the metacarpophalangeal joint, rather, is powered by the intrinsics: the interosseous muscles, which attach to the proximal phalanx; and the lumbricals, which blend into the common extensor tendon on the dorsal surface. The lumbricals originate from the flexor digitorum profundus tendon, cross the finger on its radial border, and at- tach dorsally into the common extensor ten- don. The lumbricals, thus, power metacarpophalangeal flexion as well as extension of the interphalangeal joints—especially when the metacarpophalangeal joint is flexed. The combined function of the intrinsics and the extrinsic flexor-extensor tendons gives a smooth and coordinated composite motion to the digits.
Nerves and Blood Vessels
The three major nerves of the hand are the radial, median, and ulnar nerves. The radial nerve, through its posterior interosseus branch, provides innervation to extensors of the fingers. The radial nerve also provides sensation on the radial aspect of the dorsum of the hand. The median nerve provides sensation to the thumb and index and long fingers, as well as to the radial half of the ring finger. It provides the motor innervation of all of the forearm flexor muscles, with the exception of the flexor carpi ulnaris and the ulnar aspect of the flexor digitorum profundus (that part of the muscle going to the ring and little fingers), which are supplied by the ulnar nerve. The ulnar nerve most importantly provides all of the motor innervation of most of the intrinsic muscles of the hand. The ulnar nerve also provides sensation to the ulnar aspect of the palm, to the little finger, and to the ulnar aspect of the ring finger. The lumbricals to the index and long finger and the thenar muscles, with the exception of the adductor pollicis, are supplied by the median nerve.
The radial artery enters the hand dorsally (although the pulse is more easily palpated on the volar side) and terminates in the deep palmar arch. The ulnar artery enters the hand through Guyon’s canal, at which point it is very close to the ulnar nerve. It terminates in the superficial palmar arch farther distal in the hand. In most people, these two arches communicate. This allows a patient with an injury to one of the two main arteries to maintain viability of the hand and fingers. The palmar arches supply the common digital arteries that further bifurcate into proper digital arteries. Each finger has a radial and ulnar artery that run in the volar aspect of the finger adjacent to the flexor sheath with the digital nerves.
Bones and Joints
The elbow is the junction of the distal humerus and the two bones of the forearm, the radius and ulna (Fig. 9).
Unlike the knee, at which the femur makes contact with only one of the two distal bones (the tibia but not the fibula), in the elbow joint, the humerus makes contact with bone at the ulna (a joint built for elbow flexion and extension) and the radius (a joint that allows forearm rotation). The distal humerus flares medially and laterally, forming two condyles. The condyles are covered with articular cartilage and serve as the contact points for the joint. The smaller prominences just proximal to the joint are the epicondyles. These are not cov- ered with cartilage but rather serve as the points of origin for many forearm muscles. The medial epicondyle is the site of attachment of the flexor-pronator muscle group of the forearm as well as the ulnar collateral ligament. The lateral epicondyle is the origin of the extensor muscles of the forearm.
The elbow is made up of two joints. The first, the humeroulnar joint, is hinge-like and allows flexion and extension. This joint is formed by the trochlea on the medial surface of the distal humerus and the proximal ulna. The ulna has a coronoid process anteriorly and an olecranon process posteriorly; these grasp the humeral trochlea front and back. Lateral to the trochlea is the second joint in the elbow, mating the capitellum of the humerus to the radial head. The radial head is discoid and the capitellum is hemispherical, allowing pronation and supination of the forearm. Since the radius is intimately bound to the ulna, a so-called proximal radioulnar joint is formed.
Ligaments and Soft Tissues
The bony congruity between the trochlea and the ulna provides significant stability to the elbow. This joint is nonetheless supplement- ed by the ligaments and capsule (Fig. 10).
The ulnar (or medial) collateral ligament is the primary stabilizer of the medial side of the elbow. This ligament is particularly important in the throwing athlete, who places significant forces on the elbow when cocking the arm. The radial (or lateral) collateral ligament supports the lateral side of the elbow.
The joint between the radial head and the capitellum lacks bony stability. This joint is therefore stabilized by the annular ligament. This structure, as its name implies, is a ring that surrounds the radial neck just distal to the radial head. It attaches the radius to the ulna—but since the ulna is firmly attached to the humerus, it also stabilizes the radius to the humerus.
Most of the muscles that cross the elbow joint originate from one of the two epi- condyles. Muscles on the dorsum of the fore- arm originate from the lateral epicondyle, whereas those on the volar surface originate from the medial epicondyle. The volar mus- cles are primarily flexors (Fig. 11)
and the dorsal muscles are extensors (Fig. 12).
There are three muscles that cross the elbow but originate proximal to the epicondyle. The bi- ceps originates from two points on the scapula and proximal humerus and courses on the anterior surface of the arm. Its tendon crosses the anterior aspect of the elbow joint and inserts on the proximal radius. As such, it both flexes the elbow and supinates the forearm. The brachialis muscle originates from the anterior humerus and inserts on the proximal ulna, providing elbow flexion. The triceps brachii originates from both the scapula and the posterior humerus, courses posterior to the elbow joint, and inserts on the olecranon process of the ulna, providing elbow extension.
The medial epicondyle is the origin of the “superficial” flexor muscle group. This group includes the pronator teres, the flexor carpi radialis, the palmaris longus, the flexor carpi ulnaris, and the flexor digitorum superficialis, all of which provide some stability to the medial elbow. The pronator teres allows for forearm pronation and elbow flexion. The others flex the wrist, with the exception of the flexor digitorum superficialis, whose action is on the proximal interphalangeal (fin- ger) joint. The median nerve innervates all of the superficial flexor muscles, except for the flexor carpi ulnaris, which is supplied by the ulnar nerve. The “deep” flexors of the forearm originate in the medial forearm, most of them from the ulna. This group includes the flexor digitorum profundus, the flexor pollicis longus, and the pronator quadratus, all of which are supplied by the anterior interosseous branch of the median nerve, except for the ulnar half of the flexor digitorum profundus, which is supplied by the ulnar nerve.
From the lateral epicondyle arise the ex- tensors of the wrist and hand. The superficial group includes the brachioradialis, the extensor carpi radialis longus, and the extensor carpi radialis brevis. These three muscles are known as the mobile wad of three—a name that makes sense if you pinch and shake them. The brachioradialis attaches to the distal radius and flexes the elbow; the extensor carpi radialis longus and the extensor carpi radialis brevis insert on the index and middle metacarpals, respectively, and extend the wrist. All receive innervation from the radial nerve. The lateral epicondyle is the point of origin for three other superfi- cial extensors: the extensor digitorum, the extensor digiti minimi, and the extensor carpi ulnaris. The extensor carpi ulnaris attaches to the fifth metacarpal and extends the wrist. (The insertions of the finger extensors are somewhat complex and are discussed in the hand section below.) The posterior inter- osseous nerve, a branch of the radial nerve, innervates these three superficial extensor muscles as well as the deep extensors below them. The deep extensors include the supina- tor, three muscles to the thumb (the abductor pollicis longus, the extensor pollicis longus, and the extensor pollicis brevis), and the ex- tensor indicis proprius.
Nerves and Blood Vessels
The median nerve crosses the anterior elbow, superficial to the brachialis muscle and medial to the brachial artery. The median nerve supplies the superficial flexors and travels through the carpal tunnel into the hand. In the forearm, the median nerve gives off the anterior interosseous nerve, which supplies all of the deep flexors, except the ulnar half of the flexor digitorum profundus. The ulnar nerve crosses the elbow joint just posterior to the medial epicondyle, a common site of nerve compression and irritation. When the medial elbow is hit, the superficial placement of the ulnar nerve causes it to absorb some of that energy, resulting in paresthesias (colloquially termed “hitting the funny bone”).
In the forearm, the ulnar nerve supplies the flexor carpi ulnaris and the ulnar half of the flexor digitorum profundus. Its main targets are in the hand, however. The ulnar nerve reaches the hand by way of Guyon’s canal at the wrist. It supplies the hypothenar muscles, most of the intrinsics, and the adductor pollicis. The radial nerve crosses the elbow anterior to the lateral epicondyle, where it gives off the posterior interosseous nerve. The radial nerve itself supplies the mobile wad of three. The posterior interosseous branch of the radial nerve feeds all extensor muscles distal to the mobile wad of three.
The brachial artery travels on the anterior surface of the humerus. It crosses the elbow anteriorly, where, at the level of the neck of the radius, it bifurcates into the ulnar and radial arteries. It also gives off recurrent branches that course back toward the arm both medially and laterally. The radial artery runs on the dorsum of the forearm under the brachioradialis muscle and terminates in the deep palmar arch in the hand. The ulnar artery runs on the volar surface under the su- perficial flexor muscles and terminates in the hand as the superficial palmar arch.
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