Abstract
Introduction
Knee ligament laxity and soft tissue balance are important pre- and intra- operative balancing factors in total knee arthroplasty (TKA). Laxity can be measured pre-operatively from short-leg radiographs using a stress device to apply a reproducible force to the knee, whereas intra-operative laxity is routinely measured using a navigation system in which a variable surgeon-applied force is applied. The relationship between these two methods and TKA outcome however, has not been investigated. This study aims to determine how intra-operative assessments of laxity relate to functional radiographic assessments performed on pre-operatively. We also investigate how laxity relates to short-term patient-reported outcomes.
Method
A prospective consecutive study of 60 knees was performed. Eight weeks prior to surgery, patients had a CT scan and functional radiographs captured using a Telos stress device (Metax, Germany). This device applies a force to the knee joint while bracing the hip and ankle causing either a varus or valgus response.
3D bone models were segmented from the CT scan and landmarked to generate patient specific axes and alignments. Individual bone models were registered to the 2D stressed X-rays in flexion and extension. Reference axes identified on the registered 3D bone models were used to measure the coronal plane laxity. These laxity ranges were compared with those measured by a navigation system (OMNINAV, OMNI Life Science, MA) used during surgery, and Knee Injury and Osteoarthritis Outcome Scores (KOOS) captured 6 months postoperatively.
Results
Laxity measurements were acquired from 54 patients (58 knees; 4 bilaterals). The average age was 65±15 years old and 57% (n=31) of the patients were female.
The midpoints of the laxity curves generated by Telos and navigation techniques show significant strong correlations in extension (r = 0.83, p < 0.001) and flexion (r = 0.53, p < 0.001). However, the laxity ranges measured by the two techniques did not. On average the navigation system produced significantly larger laxity range measurements than the Telos stressed x- ray technique in both extension (Nav: 8.4° ± 2.0°; Telos: 4.0° ± 2.4°; p < 0.001) and flexion (Nav: 5.0 ± 2.4; Telos: 3.0 ± 2.4; p < 0.001).
Telos-generated laxity ranges indicate that patients who have initially greater laxity in extension than flexion (laxity range difference > 2°) have significantly better 6-month pain KOOS than those who show greater laxity in flexion (laxity range difference < −2°) (p = 0.018), see Figure 1. This correlation does not hold however, when examining laxity ranges generated by the navigation system, see Figure 2.
Discussion and Conclusions
Significantly larger navigation-generated laxity ranges may be caused by: variable forces applied by the surgeon while the patient is under anaesthetic, surpassing the patient's functional limit; as well as due to the altered physical state of the knee during surgery. More sophisticated techniques to reproducibly assess intra-operative soft tissue balance may be required to accurately define laxity range. Results indicate that functional Telos-generated laxity ranges may provide unique insight into the relationship between laxity and postoperative outcomes that cannot be attained with passive navigated measurements.
