Abstract
Introduction
The mechanical classical method of knee surgical instrumentation by alignment is based on built-in compromises and is considered insufficient to ensure consistent success. Soft tissue balancing is thus now seen as necessary for optimal functional outcomes and patient satisfaction. (Matsuda 2005, Winemaker 2002). The authors have previously demonstrated that balancing can be achieved through specific strategic moves. In this study, the goal was to determine the efficacy of a given surgical algorithm and to define predictors of improved outcome. The surgical target is equilibrium of contact loads. The mechanical axis remains in neutral, however subtle variation in the joint line obliquity and posterior slope are tolerated within the literature established boundaries of +/− 3 degrees and less than 10 degrees respectively.
Methods
Data was obtained from 101 consecutive primary procedures from a single surgeon (PAM) using a PCL-retaining device. For all cases the testing methodology consisted of a sag test, heel push, drawer testing at 90 degrees, and varus-valgus laxity testing at 10 degrees of flexion. Instrumented tibial trials were used to measure the contact forces on the lateral and medial sides at 10, 30, 60 and 90 degrees of flexion. Specific releases were identified and noted based on matrix profiling after each test. Re-iteration loops were enacted until balance within 15 lbs. of difference was achieved. The data was expressed as the ratio of medial/total force (total=medial + lateral), with 0.5 being equal lateral and medial forces. This was named the Contact Load Ratio (CLR). The load distribution was expressed as a scatter graph of lateral v. medial compartmental loads (Figure 1)
Results
The number of corrections required to achieve balancing was less than five, with on average of two. The salient finding was the clustering of the Contact Load Ratio after surgical balancing was performed. As seen in Fig 1, there was increasing precision and reproducibility in achieving loads in reasonable ranges and similar in magnitude. The preliminary 3months observational data indicates a trend towards higher functionality and patient satisfaction when the loads lie within a defined sector: the safe or target balancing zone (Fig 2). This is currently defined as bracketed between 12–40 lbs of amplitude in either compartment and within 0.8 to 1.25 of ratio between the medial side and the total contact load across the joint.
Discussion
It is believed by the authors that the clinical use of load balancing algorithms can predictably restore a comparable load ratio with magnitudes within the defined safe zone. This precision however may not necessarily convert into better performance. At 65 % completion however the functional score results do indicate a definite such trend. Although a perfect load symmetry (0.5 ratio) might be intuitively desirable, the higher value of 0.52 may be concordant with the published varus / valgus ratio of 0.55, in healthy individuals (Heeserbeek 2008). The role of implant design, inflammatory laxity, sex, anticipated level of function, and age all may affect the ideal individual ratio. Further studies are necessary to further refine this concept.
