Abstract
Background
Correct placement of the acetabular cup is a crucial step in total hip replacement to achieve a satisfactory result and remains a challenge with free-hand techniques. Imageless navigation may provide a viable alternative to free-hand technique and improve placement significantly. The purpose of this project was to assess and validate intra-operative placement values for both inclination and anteversion as displayed by an imageless navigation system to post-operative measurement of cup position using high resolution CT scans.
Methods
Thirty-two subjects who underwent primary hip joint arthroplasty using imageless navigation were included. The average age was 66.5 years (range 32-87). 23 non-cemented and 9 cemented acetabular cups were implanted. The desired position for the cup was 45 degrees of inversion and 15 degrees of anteversion. A pelvic CT scan using a multi-slice CT was used to assess the position of the cup radiographically.
Results
Two subjects were excluded because of dislodgement of the tracking pin. Pearson correlation revealed a strong and significant correlation (r = 0.68; p < 0.006) for cup inclination and a moderate non-significant correlation (r = 0.53; p = 0.45) between intra-operative readings and cup placement for anteversion.
Conclusions
These findings can be explained with the possible introduction of systematic error. Even though the acquisition of anatomic landmarks is simple, they must be acquired with great precision. An error of 1cm can result in a mean anteversion error of 6 degrees and inclination error of 2.5 degrees. Whilst computer assisted surgery results in highly accurate cup placements for inclination, anteversion of the cup cannot be determined accurately.
Correct placement of the acetabular cup in total hip arthroplasty is a crucial step to achieve a satisfactory result and remains a challenge with free-hand techniques1-3. Indeed, malpositioning can induce early loosening, high wear and postoperative dislocation4-6. Various investigators have demonstrated that conventional free-hand positioning can result in a high percentage of unacceptable acetabular cup placements2,3,7,8.
Imageless navigation may provide a viable alternative to free-hand techniques and the use of mechanical guides and may improve placement significantly7,9,10. Previous authors have demonstrated that cup alignment significantly improved with the use of computer navigation3,9,11-14. Imageless computer aided navigation relies on a pelvic coordinate system which uses bony landmarks (anterior superior iliac spines and pubic tubercle) to define the anterior frontal plane15,16. These bony landmarks are determined by palpation and digitisation through the overlying soft-tissue with a metal pointer16. Manual digitisation can potentially cause measurement error which, in turn, can result in excessive tilt of the cup in the frontal plane. This is particularly problematic in obese patients where excess soft tissue can completely obscure bony landmarks. Clearly, the introduction of systematic error may lead to cup placement which differs from the intra-operative readings when using imageless navigation.
Therefore, the purpose of this study was to assess and validate intra-operative placement orientation as displayed by the navigation unit to post-operative measurement of cup position using high resolution CT scans. We hypothesised that inclination is highly accurate as the anterior superior iliac spines are easily palpable even in obese patients. In contrast, we hypothesised that anteversion is inaccurate due to the underlying soft-tissue and the difficulty in identifying the pubic tubercle.
Methods
Patient selection
Between June 2005 and December 2007, 32 patients underwent primary hip joint replacement using imageless navigation. Two patients had to be excluded because of intra-operative dislodgement of the tracking pin. The mean patient age was 66.5 ± 14 (range 28-87) years. There were 16 males (mean age 62.2 ± 12.2) and 14 females (mean age 71.4 ± 14.7. The mean weight was 85.6 ± 14 kg (range 57-112), the mean height measured 169 ± 8.6 cm (range153-186) and the average BMI was 30.04 ± 4.6 kg/m2 (range 20.9-39.5). Twenty one non-cemented and 9 cemented acetabular cups were implanted. The average size of the non-cemented cup was 53 mm (range 46-60) and averaged 54 (range 50-58) for the cemented cup. The main indication was primary osteoarthritis (n = 25), osteonecrosis (n = 5), displaced neck of femur fracture (n = 1) and failed screw fixation with head collapse after neck of femur fracture (n = 1). In 17 subjects, hip arthroplasty was performed on the right hip and 15 subjects had a left total hip arthroplasty. Surgery was performed by a single surgeon who was an experienced user of the imageless navigation system.
Sequence of Navigation
