Abstract
INTRODUCTION
In patients presenting with significant ligamentous instability/insufficiency and/or significant varus/valgus deformity of the knee, reproduction of knee alignment and soft tissue stability continues to be a difficult task to achieve. These complex primary total knee arthroplasty (TKA) candidates generally require TKA systems incorporating increasing levels of constraint due to the soft-tissue and/or bone deficiencies. In addition, achievement of “normal” gap symmetry through physiologic kinematics is challenging due to the complexity of the overall correction. Advancements in TKA design have not fully addressed the negative consequences of the increased forces between the degree of component constraint, the femoral box, and the tibial post. The purpose of this early feasibility study was to introduce the design characteristics of a primary TKA system that incorporates progressive constraint kinematics using a low profile trapezoidal femoral box, and to assess the short-term clinical and radiographic results of this patient cohort.
METHODS
We retrospectively evaluated 22 consecutive, non-selected, complex primary TKA patients with a minimum of 3-years follow-up and varus deformity of > 20 degrees or valgus deformity of >15 degrees. The Progressive Constraint Kinematics® Knee System (PCK, MAXX Orthopedics, Norristown, PA) was used and provides a variable constraint profile, from high constraint in extension to less constraint in flexion through a novel trapezoidal femoral box. We evaluated patient demographics, pre- and post-operative serial radiography, range of motion (ROM), and total Knee Society Score (KSS – total score). General descriptive statistics and paired t-Test to assess the difference between means at p <0.05 level of significance.
RESULTS
The average time to most recent follow-up was 40.5 ±3.5 months (range: 36.0 to 44.0 months). The PCK knee system had 100% survival rate at the most recent follow-up, with no reports of adverse events, subsequent corrective surgery, or revision. The average total KSS improved from 72.7 ±3.2 (range: 68 to 81) pre-operatively to 92.3 ±2.1 (range: 88 to 96) post-operatively (p < 0.001). Full post-operative arc of motion was 0 – 130° and there was no radiographic evidence of composite degradation, aseptic loosening or component malalignment.
DISCUSSION/CONCLUSION
The PCK Knee System utilizes a trapezoidal shaped femoral box, where the narrower end is located anteriorly, allowing a valgus/varus tilt of 1–4 degrees and internal/external rotation of 2–7 degrees during flexion, while maintaining necessary soft-tissue constraint during extension. This variable constraint profile allows for fully tensed collaterals in extension, with a slight reduction in collateral tension through flexion. Furthermore, the combination of the condylar anatomy, trapezoidal femoral box and tibial post allows for adequate clearance through full flexion, while facilitating slightly progressive increases in tilt and rotation, thereby maintaining knee kinematics while dampening forces transmitted through the prosthetic composite. From this feasibility study we report promising short-term clinical and radiographic results in the absence of biomechanical failure in complex primary TKA cases. We recommend continuation of the use and further research of the PCK Knee System for complex primary TKA with the ultimate goal of further determining cost effectiveness and intermediate to long-term clinical relevance.
