Every Knee Is Different: The Case for Shape Matched Total Knee Arthroplasty

Chris Jones MB, BS, FRACS (Orth)
Arthroplasty Fellow
Queen Elizabeth II Health Sciences Centre
Dalhousie University
Halifax, NS

Shane Guerin MCh, MEng, FRCSI (Tr & Orth)
Arthroplasty Fellow
Queen Elizabeth II Health Sciences Centre
Dalhousie University
Halifax, NS

Glen Richardson, M.D., MSc, FRCSC
Assistant Professor of Surgery
Queen Elizabeth II Health Sciences Centre
Dalhousie University
Halifax, NS

Michael J. Dunbar M.D., FRCSC, PhD
Professor of Biomedical Engineering
Professor of Community Health and Epidemiology
Professor of Surgery
Queen Elizabeth II Health Sciences Centre
Dalhousie University
Halifax, NS

The normal knee has a neutral mechanical alignment with a hip-knee-ankle angle of approximately zero degrees1 but most individuals tend to be either slightly varus or valgus2. The joint line in the normal knee is not perpendicular to this mechanical axis, but inclined approximately 3° such that the medial proximal tibial angle is 87° 1, 3, 4. Conventional total knee arthroplasty (TKA) does not aim to reproduce this normal, pre-disease anatomy, instead intentionally placing the joint line perpendicular to the mechanical axis. This intention is based primarily on historical theoretical biomechanical studies5-7 and studies of outdated implants8-10.

 

Furthermore conventional knee arthroplasty instrumentation only achieves a neutral mechanical axis in as few as 61% of TKA's11, and may internally malrotate the femoral component up to 8° more than a computer-assisted TKA12. A mechanical alignment of ±3° may not confer an advantage in terms of implant survival13, 14. However, when the goal of surgery is neutral alignment, mechanically malaligned knees have been shown to result in lower patient satisfaction11. Satisfaction rates following conventional TKA are between 75 and 88%, approaching 81% as study population size increases15-23. If the goal is to provide a lasting implant and high patient satisfaction, we need to look beyond conventional TKA.

Shape matching involves aligning TKA implants based on the pre-arthritic alignment of the knee. This is achieved by resecting, from each distal and posterior femoral condyle and from each tibial plateau, a thickness of bone that, after wear and saw kerf are accounted for, is equal to the thickness of the prosthesis to be implanted. As the posterior femoral condyles have been shown to be circular in sagittal profile2, preoperative magnetic resonance imaging (MRI), using proprietary software, is able to calculate bone and cartilage loss, align the joint to its pre-arthritic state and calculate appropriate implant sizing24. Final limb and component alignment can be assessed during this preoperative planning stage. Disposable patient-specific cutting blocks are then constructed using the MRI-based plan. The femoral cutting block sets the distal femoral cut and the femoral component rotation, and the tibial block sets the tibial cut and the tibial component rotation. Removal of peripheral osteophytes potentially allows medial and lateral collateral ligaments to function normally and soft tissue releases are typically not required. The cost of the MRI and the custom blocks is considerably offset by reduced inventory and instrumentation and shorter operative time25, 26.

Howell25 has presented the results of 48 shape matched TKA's implanted with patient-specific MRI-based cutting blocks. At three months postop 60% of patients rated their knee as "nearly normal" and 35% as "normal". The postoperative coronal mechanical alignment was 1.4 ±2.8° valgus with slight varus tibial component alignment and slight compensatory femoral component valgus, resulting in a joint line obliquity consistent with normal knee anatomy. Studies suggesting this alignment of the tibial component may increase failure rates8-10, 27 reviewed outdated implants and assessed alignment only with short knee films, which are inadequate for assessing coronal knee alignment28-31. RSA studies have not shown individual TKA component alignment to be associated with an increased risk of prosthetic loosening32-34. In a pilot study of four patients managed with patient-specific MRI-based cutting blocks to shape matched aligned TKA's, Klatt et al.35 reported problems with malalignment. The cutting blocks in two of the four patients were later found to be inaccurate due to an MRI alignment error that has subsequently been resolved, and the authors used short knee films, which are inadequate at assessing coronal knee alignment28-31.

Howell36 suggests that the traditional concept of a hypoplastic lateral femoral condyle in the valgus knee does not occur, and that instead the supposedly hypoplastic lateral femoral condyle of the valgus knee is of a similar radius to the medial femoral condyle, but translated proximally. Shape matching a valgus knee results in pre-disease alignment and as such, varus and valgus knees are amenable to shape matched alignment25. However, shape matched alignment is not suitable for every knee. The pre-disease alignment of the knee cannot be accurately assessed in cases of post-traumatic arthritis and in cases of where significant bone destruction has occurred, such as severe inflammatory or post-septic arthritis. The perioperative experience and outcome of shape matched TKA for knees exceeding 15° of coronal malalignment has not been reported. Nonetheless, shape matched alignment is suitable for the vast majority of patients undergoing TKA. As with all procedures, common sense must prevail and when performing a shape matched TKA, the surgeon should ensure that bone cuts are of an appropriate thickness for the planned prosthesis, accounting for wear and saw kerf.

Unlike conventional total knee arthroplasty, shape matched TKA aligns both the limb and the implants to the individual patient's pre-arthritic state. By considering each patient as an individual rather than an average, shape matched TKA aims to improve patient satisfaction without compromising implant longevity. Shape matched TKA as a concept needs further validation and clearly more research is required. As such, shape matched TKA should be introduced with caution in a phased innovation fashion37.

Disclosures:
Dr. Dunbar is a consultant for and receives royalties from Stryker, is an editorial board member for Journal of Bone and Joint Surgery (British) and The Journal of Knee Surgery, and is a board member of the Medical Advisory Board of the Arthritis Society of Canada. Institutional support is received from Stryker, Zimmer, DePuy and Smith & Nephew.

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