Figure. Standing anteroposterior radiograph of the knees.
A 57-year-old woman presents with bilateral knee pain. Standing anteroposterior radiograph (Figure) shows varus deformities of both knees. The patient is scheduled for a total knee replacement.
Which of the following describes the mechanical axis upon which total knee alignment is based?
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The optimal alignment for total knee replacement is widely accepted to be within 3 degrees of the neutral mechanical axis. The mechanical axis of the lower extremity is determined by a line drawn from the center of the femoral head to the center of the ankle joint. This alignment is achieved by making precise bone cuts and adequate soft tissue release. The femoral and tibial cuts are made perpendicular to the mechanical axis of the leg. On the femoral side, an intramedullary cutting jig is used to make the distal femoral cut. On the tibial side, bone resection is greater on the high side (lateral tibia in a varus knee), which is perpendicular to the mechanic axis. Soft tissue release is often necessary depending on the alignment of the arthritic knee (varus or valgus). In a varus knee, the medial collateral ligament may be contracted and should be lifted off the medial tibia to open the medial tibiofemoral gap.
Recent literature suggests that implanting a total knee in a neutral mechanical axis may not be as important as once thought. Abdel et al found that postoperative alignment within 3 degrees of a neutral mechanical axis does not improve implant survival or patient function. This evidence suggests that the benefits of computer navigation to restore precise alignment in total knee replacement may not be cost-effective.1,2
Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor-in-chief of the Journal of Orthopedics for Physician Assistants (JOPA).
- Abdel MP, Ollivier M, Parratte S, Trousdale R, Berry D, Pagnano M. Effect of postoperative mechanical axis alignment on survival and functional outcomes of modern total knee arthroplasties with cement: a concise follow-up at 20 years. J Bone Joint Surg Am. 2018;100(6):472-478.
- Lording T, Lustig S, Neyret P. Coronal alignment after total knee arthroplasty. EFORT Open Rev. 2016;1(1):12-17.