Summary

An analysis of how corrective and non-corrective joint laxity can be integrated into pre-surgical planning

Abstract

Introduction

Pre-surgical planning in total knee arthroplasty (TKA) is based on static non-functional imaging (CT or MRI). Component alignment is determined prior to any assessment of clinical soft tissue laxity. This has lead to assumptions of correctability. The aim of this study was to determine whether functional radiology capture could complement pre-surgical planning by identifying non-predictable patient variation in laxity.

METHODS: Pre-operative CT’s, standing radiographs and functional radiographs assessing coronal laxity at 20° flexion were collected for 54 patients. Varus/valgus laxity was assessed using the TELOS stress device. The varus/valgus load was incrementally increased to either a maximum load of 150N or until the patient could not tolerate the discomfort. Radiographs were taken whilst the knee was held in the stressed position. CT scans were segmented and bony anatomical points landmarked with 2D-3D pose estimations being performed.

RESULTS SECTION: The mean coronal alignment in CT and standing radiographs was 3.1° varus (SD, 5.5°) and 3.7° varus (SD, 6.3°) respectively. Of this 54 patients, 41 knees were varus aligned and 13 valgus aligned in weight bearing and CT positions. The varus group had means of 5.5° (SD, 3.2°) varus in CT and 6.6° (SD, 4.0°) varus in standing, while the valgus group had means of 4.5° (SD, 2.6°) valgus in CT and 5.2° (SD, 2.9°) valgus in standing. The mean alignment change from CT to standing was 1.6° (SD, 1.4°) in varus group and 1.4° (SD, 1.2°) in valgus group. This suggests the degree of deformity increases under weightbearing.

Varus and valgus limits for each knee were determined using stressed radiographs at 20° flexion to define the laxity envelope. The envelope for varus knees ranged from 7o valgus to 17o varus, with a mean of 4.1o (SD, 2.5o) while the envelope for valgus knees ranged from 13.3o valgus to 2.0o varus, with a mean of 5.5o (SD, 2.2o). There were 31 out of the 54 patients had a varus/valgus laxity envelope that passed through neutral mechanical alignment ±3°. Using ±3° varus valgus alignment as an indicator of correctable deformity, 43% of the patients were not considered to have a correctable deformity.

As determined by laxity limits, the CT and standing alignments were not well centered within their functional radiology groups. In varus group, 19 (46.4%) had the standing alignment near the varus limit and 4 (9.8%) near the valgus limit, while in valgus group, 1 (7.7%) had the standing alignment near the varus limit and 6 (46.2%) near the valgus limit. In total, 56% of the knees did not have their standing alignment well centered on their functional laxity limits.

DISCUSSION: Varus valgus laxity in TKA appears to be subject specific and separate from static radiological parameters. Surgical planning without reproducible clinical assessments of coronal laxity may not be sufficient to obtain a balanced TKA while avoiding ligament releases.

CONCLUSION: Functional radiographs may be a viable method to individualise and refine the surgical plan in TKA on a per patient basis, incorporating objective information normally only available during the surgery itself.