Abstract
Introduction
Balancing at surgery is important for clinical outcome in terms of pain relief, flexion range, and function. The methodology usually involves making bone cuts to achieve correct leg alignment, and then obtaining equal gaps in extension and flexion using spacer blocks or tensor devices. In this study, we describe a method for quantifying balancing throughout the flexion range and show the effect of different surgical corrections from an unbalanced to a balanced state. In this way, we quantified how accurately balancing could be achieved within the practical time frame of a surgical procedure.
Methods
Data was obtained from 80 primary procedures using a PCL-retaining device. Initial bone cuts were made using navigation. Instrumented tibial trials were used to measure the contact forces and locations on the lateral and medial sides. Video/audio recordings were made of all aspects of the surgeries. The initial balancing was recorded during the Heel Push Test, namely the lateral and medial contact forces for the flexion range. The data was expressed as medial/total force ratio (total=medial + lateral), with 0.5 being equal lateral and medial forces. Surgical corrections to correct the specific imbalance pattern, determined from previous research, were carried out. The Heel Push Test was repeated after each correction and at final balancing.
Results
The initial balancing before correction showed that although the average ratio was 0.52±0.27 from 0–90 degrees, the data was scattered between 0.0 (lateral force only) and 1.0 (medial force only). The most common surgical corrections used to achieve balancing were: soft-tissue releases (49), changes in tibial insert thickness (27), bone adjustments (15), tibial rotational adjustments (7).
In 84% of the cases, 0–2 corrections were needed to obtain balancing (Range: 0–5). 80% of the cases in early flexion (0–30 degrees) were balanced within 15% of the balanced state (79 % for 30–60 deg of flexion, 77% for 60–90 deg of flexion). The mean ratio for all flexion angles was 0.52 with standard deviation of 0.16.
Discussion
By following a set of logical steps, accurate balancing was achieved in the majority of cases with only 1–2 Surgical Corrections being necessary. Corrections of both bone cuts and soft tissue were applicable. The most important range for balancing was early in flexion. There was no target value of the total forces because the ligament stiffnesses varied substantially between patients. Further, while a 0.5 ratio was aimed for, we expect that the ideal value will be in the region of 0.6, higher medial forces than lateral, in line with functional forces. Studies are now underway to determine the effect of balancing on the functional results.
