What Factors Are Associated With Femoral Component Internal Rotation in TKA Using the Gap Balancing Technique?

Abstract

Background: When using the gap-balancing technique for TKA, excessive medial release and varus proximal tibial resection can be associated with internal rotation of the femoral component. Previous studies have evaluated the causes of femoral component rotational alignment with a separate factor analysis using unadjusted statistical methods, which might result in treatment effects being attributed to confounding variables.

Questions/purposes: (1) What pre- and intraoperative factors are associated with internal rotation of the femoral component in TKA using the gap balancing technique? (2) To what degree does femoral component rotation as defined by the navigation system differ from rotation as measured by postoperative CT?

Methods: Three hundred seventy-seven knees that underwent computer-assisted primary TKA attributable to degenerative osteoarthritis with varus or mild valgus alignment in which medial soft tissue release was performed, and those with preoperative radiographs including preoperative CT between October 2007 and June 2014 were included in the study. To achieve a balanced mediolateral gap, the structures released during each medial release step were as follows: Step 1, deep medial collateral ligament (MCL); Step 2, superficial MCL (proximal, above the pes anserine tendon) and semimembranosus tendon; and Step 3, the superficial MCL (distal, below the pes anserine tendon). Knees with internal rotation of the femoral component, which was directed by navigation, to achieve a rectangular mediolateral flexion gap were considered cases, and knees without internally rotated femoral components were considered controls. Univariable analysis of the variables (age, sex, BMI, operated side, preoperative hip-knee-ankle angle, preoperative medial proximal tibial angle, preoperative rotation degree of the clinical transepicondylar axis [TEA] relative to the posterior condylar axis [PCA], coronal angle of resected tibia, resection of the posterior cruciate ligament, type of prosthesis, and extent of medial release) of cases and controls was performed, followed by a multivariable logistic regression analysis on those factors where p equals 0.15 or less. For an evaluation of navigation error, 88 knees that underwent postoperative CT were analyzed. Postoperative CT scans were obtained for patients with unexplained pain or stiffness after the operations. Using the paired t-test and Pearson's correlation analysis, the postoperative TEA-PCA measured with postoperative CT was compared with theoretical TEA-PCA, which was calculated with preoperative TEA-PCA and actual femoral component rotation checked by the navigation system.

Results: After controlling for a relevant confounding variable such as postoperative hip-knee-ankle angle, we found that the extent of medial release (Step 1 as reference; Step 2: odds ratio [OR], 5.7, [95% CI, 2.2-15]; Step 3: OR, 22, [95% CI, 7.8-62], p < 0.001) was the only factor we identified that was associated with internal rotation of the femoral component. With the numbers available, we found no difference between the mean theoretical postoperative TEA-PCA and the postoperative TEA-PCA measured using postoperative CT (4.8° ± 2.7º versus 5.0° ± 2.3º; mean difference, 0.2° ± 1.5º; p = 0.160).

Conclusions: Extent of medial release was the only factor we identified that was associated with internal rotation of the femoral component in gap-balancing TKA. To avoid internal rotation of the femoral component, we recommend a carefully subdivided medial-releasing technique, especially for the superficial MCL because once the superficial MCL has been completely released it cannot easily be restored.

Level of evidence: Level III, therapeutic study.

Fig. 1A–C

The possible causes of an internally rotated femoral component are shown. (A) In the varus knee, some medial release frequently is required. Adequate medial release results in a rectangular mediolateral flexion gap, which is parallel to the transepicondylar axis. (B) An overly performed medial release will result in an internally rotated femoral component relative to the transepicondylar axis. (C) Inadvertent varus resection of the proximal tibia will cause internal rotation of the femoral component.

Fig. 2

The study flow diagram is shown. RA = rheumatoid arthritis.

Fig. 3A–C

Important navigation procedures are shown. (A) After registration of bony landmarks and hip, knee, and ankle centers, the proximal tibial resection was performed. Varus-valgus angle of the resected tibia was checked (asterisk). (B) In the femoral planning step, to achieve a rectangular mediolateral gap in knee extension and flexion, varus-valgus angle and rotation (dagger) of the femoral component are adjusted with navigation dictation. The numbers displayed next to femurs and tibias represent expected amounts of bone resection. (C) After distal femoral resection, the rotational position of the AP femoral cutting jig on the distal femur (double dagger), which is positioned manually, is displayed in real time. This represents actual femoral component rotation. In this case, the femoral component is in 2º external rotation relative to the posterior condylar axis. Green = this value is as planned; Ext. = extension.

Fig. 4

The medial release steps are shown. Removal of medial osteophytes and release of tibial insertions of the deep MCL and posterior oblique ligament are performed in Step 1. If further medial release was required, the proximal insertion of the superficial MCL (above the pes anserine tendon) and anterior arm of the semimembranosus were released (Step 2). In the final step, Step 3, the distal insertion of the superficial MCL (below pes anserine tendon) was released. MCL = medial collateral ligament.

Fig. 5A–C

Comparisons between the theoretical postoperative TEA–PCA and postoperative TEA–PCA measured using postoperative CT are shown. (A) Preoperative TEA–PCA is measured using preoperative CT, and (B) postoperative TEA–PCA is measured using postoperative CT. (C) Theoretical postoperative TEA–PCA is calculated by subtracting the preoperative TEA–PCA from the rotational position of the AP femoral cutting jig on the distal femur (actual femoral component rotation [F-Rot]; Fig. 3C). For example, with 5º external rotation for preoperative TEA–PCA and 2º external rotation for actual femoral component rotation, the theoretical postoperative TEA–PCA is 3º external rotation. If the theoretical postoperative TEA–PCA is significantly different using the postoperative TEA–PCA, which was measured using postoperative CT, then this indicates an error during the navigation procedure. TEA = transepicondylar axis; PCA = posterior condylar axis; Ext. = extension.