Bone & Joint 360 Vol. 5, No. 4 Feature

Total knee arthroplasty at a crossroads

Khosrow Sehat

Knee

Total knee arthroplasty (TKA) surgery faces a dilemma. The established method of alignment to the mechanical axis results in excellent implant longevity, but with less than ideal clinical functional and pain outcomes for some patients. A relatively recent development has been the concept of TKA implant positioning according to individual patient anatomy (also known as ‘kinematic’ alignment). There is evidence that anatomic individualised TKA implant positioning results in markedly improved functional outcomes. However, the methods published so far involve implant positioning that is not consistently aligned to the mechanical axis. Therefore, there is some justified concern that improved function is being achieved at the expense of compromised, long-term durability.

Conventional TKA creates resections according to the mechanical axis. Intra-operative releases and post-operative physiotherapy adapt the soft tissue to comply with the implant position and biomechanics. The outcome of this process can be variable. Anatomic individualised TKA ‘resurfaces’ the knee articulation so that the implants are in congruity with soft-tissue biomechanics at the onset. This means less need for soft tissue adaptation and less likelihood of stalled rehabilitation.

The rationale behind the concept of anatomic individualised TKA is becoming more widely accepted, and the ‘best of both worlds’ can be achieved (Table I).

Table I. Anatomic individualised total knee arthroplasty: key points
• Conventional TKA alignment results in good implant durability, but with limited functional outcome. Greater anatomical alignment improves outcome.
• Conventional TKA alignment makes several approximations and relies on soft tissue adaptation, which may contribute to its limitations.
• Anatomical alignment ‘resurfaces’ the knee and achieves better congruency between soft tissue and implant biomechanics, and hence an improved functional outcome.
• However, the methods for anatomical alignment previously described involve using symmetric implants to resurface an asymmetric femur. This results in oblique implant positioning, raising concerns that durability may be compromised.
• The salient features of anatomical alignment that correspond better with soft tissue biomechanics include:
◦ Aligning to the natural coronal axis of the limb within a safe range, rather than the neutral mechanical axis
◦ Aligning implants to the anatomical distal femoral axis of rotation
◦ Observing the differing levels of the medial and lateral joint lines
◦ Making allowances for cartilage wear in measuring resections
◦ Maintaining the spatial relationship between lateral femoral condyle and patella
◦ Reproducing the anatomical posterior femoral offset and tibial plateau sagittal slope
◦ Overall, maintaining the spatial relationship between the femur, tibia and patella and hence minimising the requirement for soft tissue releases and post-operative adaptation
• An ideal mix of both considerations would combine stable implant positioning with the beneficial features of anatomical alignment.

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