THE EFFECT OF MEDIAL UNICOMPARTMENTAL KNEE ARTHROPLASTY ON KINEMATICS, CONTACT PATTERNS AND LATERAL COMPARTMENT DYNAMIC JOINT SPACE

INTRODUCTION

Controversy exists regarding the ability of unicompartmental knee arthroplasty (UKA) to restore native knee kinematics, with some studies suggesting native kinematics are restored in most or all patients after UKA^1–3, while others indicate UKA fails to restore native knee kinematics^4,5. Previous analysis of UKA articular contact kinematics focused on the replaced compartment^2,5, neglecting to assess the effects of the arthroplasty on the contralateral compartment which may provide insight to future pathology such as accelerated degeneration due to overload⁶ or a change in the location of cartilage contact⁷. The purpose of this study was to assess the ability of medial UKA to restore native knee kinematics, contact patterns, and lateral compartment dynamic joint space. We hypothesized that medial UKA restores knee kinematics, compartmental contact patterns, and lateral compartment dynamic joint space.

METHODS

Six patients who received fixed-bearing medial UKA consented to participate in this IRB-approved study. All patients (4 M, 2 F; average age 62 ± 6 years) completed pre-surgical (3 weeks before) and post-surgical (7±2 months) testing. Synchronized biplane radiographs were collected at 100 images per second during three repetitions of a chair rise movement (Figure 1). Motion of the femur, tibia, and implants were tracked using an automated volumetric model-based tracking process that matches subject-specific 3D models of the bones and prostheses to the biplane radiographs with sub-millimeter accuracy⁸. Anatomic coordinate systems were created within the femur and tibia⁹ and used to calculate tibiofemoral kinematics¹⁰. Additional outcome measures included the center of contact in the medial and lateral compartments, and the lateral compartment dynamic joint space (i.e. the distance between subchondral bone surfaces)¹¹. The results of the three movement trials were averaged for each knee in each test session. All outcome measures were interpolated at 5° increments of knee extension (Figure 2). The average differences between knees at corresponding flexion angles were analyzed using paired t-tests with significance set at p < 0.05.

RESULTS

The UKA knee was in 5.3° more varus than the contralateral knee prior to surgery (p=0.005). After surgery, the UKA knee was in 4.9° more valgus than before surgery (p=0.005). The UKA knee was 4.3° more externally rotated than the contralateral knee post-surgery (p=0.05) (Table 1). No significant differences were observed between knees or pre- to post-surgery in lateral compartment dynamic joint space or the center of contact in the medial and lateral tibia compartments (Table 1).

DISCUSSION

These results suggest that medial UKA can restore native knee varus without significantly altering lateral compartment joint space or contact location during the chair rise movement.

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