Abstract
The purpose of balancing in total knee surgery is to achieve smooth tracking of the knee over a full range of flexion without excessive looseness or tightness on either the lateral or medial sides. Balancing is controlled by the alignment of the bone cuts, the soft tissue envelope, and the constraint of the total knee. Recently, Instrumented Tibial Trials (OrthoSensor) which measure and display the location and magnitude of the forces on the lateral and medial condyles, have been introduced, offering the possibly of predictive and quantitative balancing. This paper presents the results of experiments on 10 lower limb specimens, where the effects of altering the bone cuts or the femoral component size were measured.
A special leg mounting rig was fixed to a standard operating table. A boot was strapped to the foot, and the boot tracked along a horizontal rail to allow flexion-extension. The initial bone cuts were carried out by measured resection using a navigation system. The trial femoral component and the instrumented tibial trial were inserted, and the following tests carried out:
Sag Test; foot lifted up, the trial thickness chosen to produce zero flexion.
Heel Push Test; heel moved towards body to maximum flexion.
Varus-Valgus Test, AP and IXR Tests were also carried out, but not discussed here.
For an initial state of the knee, close to balanced, the lateral and medial contact forces were recorded for the full flexion range. The mean value of the contact forces per condyle was 77.4N, the mean in early flexion (0–60 deg) was 94.2N, and the mean in late flexion (60–120 deg) was 55.7N. The difference was due to the effect of the weight of the leg. One of the following Surgical Variables was then implemented, and the contact forces again recorded.
- 1.
Distal femoral cut; 2 mm resection (2 mm increase in insert thickness to preserve extension)
- 2.
Tibial frontal varus, 2 mm lateral stuffing
- 3.
Tibial frontal valgus, 2 mm medial stuffing
- 4.
Tibial slope angle increase (5 deg baseline); +2 degrees
- 5.
Tibial slope angle decrease (5 deg baseline); −2 degrees
- 6.
Increase in AP size of femoral component (3 mm)
The differences between the condyle force readings before and after the Surgical Variable were calculated for low and high angular ranges. The mean values for the 10 knees of the differences of the above Surgical Variables from the initial balanced state are shown in the chart.
From literature data, the mean tension increase in one collateral ligament is close to 25N/mm up to the toe of the load-elongation graph, and 50N/mm after the toe. Hence in the initial balanced state, the collateral ligaments were elongated by 2–4 mm producing pretension. From the Surgical Variables data, up to 2 mm/2 deg change in bone cuts (or 3 mm femcom change), and collateral ligament releases up to 2 mm, would correct from any unbalanced state to a balanced state.
This data provides useful guidelines for the use of the Instrumented Tibial Trials at surgery, in terms of bone cut adjustments and ligament releases.
