Review Question - QID 219554

This patient has a mildly varus-aligned knee as the MAD is medial to the knee joint and an MPTA of 84°. In the setting of this mild varus deformity, osteophyte removal and partial deep MCL release are the most likely expected steps to appropriately balance this knee (Answer 5).

Long-leg standing radiographs play a crucial role in understanding deformities in the knee and surgical planning for a TKA. Important basic coronal measurements to consider include the mechanical axis deviation (MAD), the medial proximal tibial angle (MPTA), and the lateral distal femoral angle (LDFA). An axis from the center of the femoral head to the center of the ankle creates the mechanical axis. A deviation medial to the center of the knee joint (more than 8 mm) or lateral to the center of the knee joint will determine a varus or valgus deformity, respectively (Illustration A). The MPTA and LDFA are used to determine if the deformity originates from the tibia or femur, respectively. A normal LDFA (mLDFA denoting measurements based on the mechanical axis of the femur) and MPTA is 87° (85-90°) (Illustration B). Medial structures will need to be released to coronally balance a varus knee as the medial side of the knee is tight relative to the lateral side. The basic sequential steps of a medial release involve medial osteophyte removal, deep MCL release, posteromedial corner release, medial tibial reduction osteotomy, and finally partial or complete superficial MCL release. With mild varus deformities, one can expect to perform only mild releases (osteophyte removal and partial deep MCL release) to appropriately balance the knee.

Sajjadi et al. performed a prospective study of preoperative radiographs for patients undergoing TKA. The authors aimed to assess the ability of alignment measurements to predict medial soft tissue releases in varus knees. The varus angle (VA), medial proximal tibial angle (MPTA), and joint line convergence angle (CA) were predictive of the amount of medial release that was required during the procedure. In particular, a preoperative VA greater than 19° and MPTA less than 81°, is predictive of requiring a stage 3 or 4 MCL release (partial or full release of superficial MCL). The mean MPTA requiring a stage 1 medial release (osteophyte removal with release of deep MCL and posteromedial capsule) was 84°. The authors concluded that VA and MPTA are useful in predicting medial soft tissue releases in varus knees undergoing TKA.

Sculco et al. published a JAAOS review of managing extra-articular deformities during TKA procedures. The authors emphasize the importance of measuring the MAD, MPTA, lateral distal femoral angle (LDFA), and CA for coronal alignment and surgical planning on full-length radiographs. Intra-articular compensation during TKA can be achieved for extra-articular coronal plane deformities less than 20° on the femoral side or less than 30° on the tibial side, as long as proposed resections do not violate collateral ligament insertions. For deformities outside these limitations, the authors recommend an extra-articular osteotomy to balance the TKA.

Illustrations:
Illustration A depicts the mechanical axis deviation from the center of the knee joint. A more medial deviation (more than 8 mm) indicates a varus deformity and a lateral deviation indicates a valgus deformity.
Illustration B shows some standard measurements for coronal plane alignment that can be obtained from full-length standing radiographs.

Incorrect Answers:
Answer 1: These steps would be expected for balancing a valgus deformity as the lateral side of the knee is tight. Specifically, iliotibial band release is indicated for a valgus knee that is tighter in extension. Based on the MAD and MPTA, this patient has a mild varus deformity.
Answer 2: Complete superficial MCL release is indicated in severe varus deformities (MPTA < 81°) and is typically the last structure to be released if needed. In a patient with more mild varus deformity (MPTA of 84°), the surgeon should not expect to have to release superficial MCL.
Answer 3: Closing wedge osteotomies with concomitant TKA is indicated for extra-articular deformities that can not be managed with intra-articular corrections. Typically, a deformity greater than 20° within the distal fourth of the femur or proximal fourth of the tibia is managed with a closing wedge osteotomy with long-stemmed TKA components. The present patient's deformity does not require extra-articular deformity correction.
Answer 4: These steps would be expected for balancing a valgus deformity as the lateral side of the knee is tight. Specifically, the release of the popliteus is indicated for a valgus knee that is tighter in flexion. Based on the MAD and MPTA, this patient has a mild varus deformity.