Persistent hindfoot valgus causes lateral deviation of weightbearing axis after total knee arthroplasty

Abstract

Background: The weightbearing axis of the limb goes from the pelvis to the ground and includes the hindfoot. However, the influence of hindfoot alignment on mechanical axis and overall limb alignment after TKA is unclear.

Questions/purposes: We therefore addressed the following questions: (1) Does hindfoot alignment change after TKA for knee osteoarthritis? (2) What factors influence hindfoot alignment after TKA? (3) Does deviation of the mechanical axis from the center of the knee vary when the hindfoot is considered (ground mechanical axis) as compared with when it is not (conventional mechanical axis) after TKA?

Patients and methods: We prospectively reviewed the radiographs of 125 patients who underwent 165 TKAs. We evaluated the preoperative and postoperative hip-knee-ankle (HKA) angle, conventional mechanical axis deviation, ground mechanical axis (mechanical axis which included the hindfoot) deviation, and tibiocalcaneal angle.

Results: The preoperative hindfoot valgus alignment decreased after TKA. Preoperative hindfoot alignment was the only factor that influenced hindfoot alignment after TKA. The conventional mechanical axis showed less deviation from the center of the knee when compared with ground mechanical axis which showed lateral deviation after TKA. Despite limb alignment being restored to within 3° of neutral of the conventional mechanical axis after TKA, 29% of limbs had a postoperative ground mechanical axis deviation of 10 mm or greater.

Conclusions: Accurate restoration of limb alignment after TKA may be associated with persistent hindfoot valgus alignment with the ground mechanical axis passing lateral to the center of the knee. This could have implications on long-term survival of the implant owing to possible excessive loading laterally and needs further investigation.

Figures

Fig. 1A–D

(A) A preoperative hip to ankle full-length standing radiograph shows a left knee varus arthritic deformity of 16° (HKA angle = 164°). The conventional (straight line) and ground (dotted line) mechanical axes are overlapping with no difference in deviation. (B) A preoperative Cobey’s view of the ankle of the same patient shows hindfoot alignment of 6° valgus. (C) A postoperative hip to ankle full-length standing radiograph after TKA shows a fully corrected left knee alignment (HKA angle = 180°). The conventional mechanical axis (straight line) passes through the center of the knee whereas the ground mechanical axis (dotted line) passes approximately 9.5 mm lateral to the center of the knee. (D) A postoperative Cobey’s view of the ankle shows a hindfoot alignment of 4° valgus.

Fig. 2

A comparison of the median preoperative and postoperative TCAs based on knee deformity subgroups (HKA angle) is shown. p Values are shown in brackets.

Fig. 3

A comparison of the median preoperative and postoperative CMAD and GMAD based on knee deformity subgroups (HKA angle) is shown. p Values are shown in brackets. Preop = preoperative; Postop = postoperative.

Fig. 4

Distribution of limbs based on postoperative GMAD in knees with postoperative alignment within 1° and 3° from neutral is shown.