CLINICAL VALIDATION OF A NOVEL SPRING LOADED TENSIONING DEVICE AND COMPUTER ASSISTED NAVIGATION

Abstract

The Vector Vision CT free navigation system of Brain-Lab (Heimstetten, Germany) for total knee arthroplasty incorporates a ligament balancing feature which allows recording of flexion extension gaps and clinical alignment [3]. Since routine spacer blocks do not necessarily load the joint space symmetrically, if either the bone cuts are asymmetric or the ligaments are not evenly balanced, a tensioning device that applies a constant load to the medial and lateral joint space separately and which can collapse or expand on each side independently should be able to provide a better evaluation of ligament tension and allow the computer software to better plan the appropriate bone cuts or ligament release. The tensioning device comprises 2 linked plates contacting the femur and tibia separated by two independent springs in the medial and lateral compartments. It can be positioned precisely in the joint with the navigation system and, with respect to spacer blocks, this device was designed in order to allow a dynamic evaluation of joint stiffness during all range of motion and, thanks to it’s reduced dimensions, with patella in situ. The springs apply a consistent known force on the compartments, while at the same time the gap produced by the applied forces is measured by the navigation. This study integrates previous article [1] on the validation of the tensioning device and reports the first phase of the clinical validation of the tensioning device, including first qualitative comparison with standard navigated technique and consideration on the use of the device.

A spring loaded mechanical device was designed with a constant 6kg load in the springs for each compartment. For the clinical evaluation, the device was inserted into flexion and extension spaces after the tibial cut was performed in routine computer assisted total knee arthroplasty. The gap produced by the applied forces is measured, by the navigation system, as the distance between tibial cut and most distal points on condyles in extension or most posterior points on condyles in flexion. Under the same conditions a set of solid spacer blocks were inserted to obtain a gap able to balance the knee according to surgeon’s sensitivity. This gap was used as reference and compared with the gap obtained with the spring device. The clinical evaluation was performed in order to determine whether there was a difference between the gaps as indicated by both the tensioning device and the spacer blocks. Five experienced orthopaedic surgeons were involved in a randomized study producing 58 complete data sets. Eight measurements (medial and lateral gap, in flexion and extension for tensioning device and spacer blocks) were taken intra-operatively using the ligament balancing features of the VectorVision system. Because many of the measurements were not normally distributed, nonparametric statistical tests (Mann-Whitney and Wilcoxon) were chosen to look for statistical differences, looking for differences in medians and ranking of data instead of differences in averages and distributions. Repeatability of measurements performed with the spring device was defined as the occurrence of same values obtained with the device and by surgeons with spacer blocks. Since the uncertainty in the measurements, due to optical navigation system is near, differences within a range of ±1mm were considered the same value, moreover data within a range of ±2mm were considered a positive result. This data was analyzed with Minitab software version 13.31.

Results are reported in Tab.1.

Results: extension Flexion compartment Medial lateral medial lateral average difference 0.2 (±2.5 mm) 0.0 (±3.6 mm) 1.3 (±2.1 mm) 1.4 (±2.5 mm) number of cases 57 100 57 100 57 100 57 100 median ±1mm 30 52.6 24 42.1 34 59.6 23 40.4 median ±2mm 43 75.4 36 63.2 47 82.5 36 63.2 Table.1. Average difference between gaps obtained with tensioning device and spacer blocks, and related occurrences in medial\lateral compartments inflexion and extension. Difference between the tensioning device and the spacer blocks in flexion is 0.2mm medially and 0.0mm laterally, while in flexion it is 1.3mm medially and 1.6mm laterally, moreover the alignments of resulting femoral cuts obtained with spring device can be considered the same of the alignments obtained with spacer blocks (difference is < 1°). Data, summarized in Tab.1, highlight that knee has a different behaviour in flexion and extension. Applying the same force with the tensioning device the resulting gap in extension (10mm medial, 10.5mm lateral) is lower than the one flexion (10.5mm medial, 12.5 lateral). The percentage of values around the average in the range of 1mm is 52.6% – 59.6% medially and 40.4% – 42.1% laterally, showing a higher variability on lateral compartment, while the percentage of values in a range of 2mm is75.4% – 82.5% medially and 63.2% laterally, confirming the variability.

Determining the soft tissue balance in total knee replacement is important to proper reconstruction. Traditional spacer blocks are unable to load the medial and lateral joint space independently which may compromise the surgical plan. A tensioning device which loads the-joint space independently with a constant load should theoretically allow proper planning of the bone resections and ligament releases during reconstructive surgery. The spring loaded tensor device coupled with image navigation and compared to independent spacer blocks performed by different surgeons revealed that there is no statistical difference between the gaps obtained with spacer blocks and tensioning device in extension, while in flexion there is an average difference 1.4mm, it revealed also that there was greater surgeon variability in the use of spacer blocks compared to the tensioning device. Furthermore, the device produced results that were similar to the results obtained by the spacer blocks especially when surgeon’s variation in technique was taken into account. The use of a joint tensioning device, coupled with computer assisted surgery, allows planning appropriate bone resection and ligament releases to produce matched medial and lateral joint spaces in flexion and extension. There were no reports, either inter-operatively or post-operatively, of any complications or adverse events nor any malfunctions of the device. On a number of occasions it was felt necessary to perform additional bone resections to allow the insertion of the spring device. However this should be considered as a normal part of TKA and of the inter-operative decision making process. This data also revealed, as would be expected clinically that the joint space is less in extension than in flexion after the tibial cut is performed [3]; surgeons with the help of spacer blocks apply less force in flexion in order to obtain the same gap during range of motion, the spring device, applying always the same force, is opening the joint more in flexion. Furthermore, when evaluating the lateral joint space, the tensioning device has a greater variability than the spacer blocks [4]. In this series of patients, a spring loaded tension device decreased surgeon variability in the assessment of ligament tension and, when coupled with computer navigation, allowed the surgeons to appropriately plan the femoral resection to create balanced flexion and extension spaces.