Trust in the accuracy and precision of any tool used in surgery is critical to its clinical utility. This is perhaps even more the case for costly computer-aided devices such as imageless CASs where the surgeon is relying on the data provided by the system to achieve his/her goal of component orientation. We asked the following question: how accurate and precise are the values for acetabular cup inclination and anteversion measured by an imageless CAS as compared with those measured using postoperative CT scans?
We first emphasize the data reported here are only directly applicable to the specific imageless CAS used and for the specific technique used in palpating and registering the bony landmarks in a select patient population without substantial truncal obesity. Second, this study is not a comparison of manual and CAS techniques. There are other published studies addressing the issue of safe zones and outliers [10, 15, 18-20, 24, 32, 39]. Instead, regardless of the target orientation chosen, we focused on the accuracy and precision of the values obtained. Third, we did not address the more difficult question of what the specific target orientation of the acetabular component should be. There is substantial variation in patients' pelvic orientation during activities of daily living . Determining the optimal individual target position for each patient may not be feasible, but future research should consider this problem. Finally, there is also concern that there is no absolute gold standard against which we can compare the values obtained by the CAS. We chose postoperative CT scans for comparison, but there may be errors in obtaining the values of inclination and anteversion from these scans. CT scan measurement as a method of determining postoperative cup position represents the most contemporary measurement technique. With the ability to precisely reference anatomic landmarks, CT scanning has emerged as the gold standard in determination of postoperative acetabular cup position [8, 14, 15, 19, 26, 38, 40]. CT protocols, using artifact suppression software, effectively reduce image distortion and acquire precise data points. We created 3D models of the pelvis and points of registration precisely defined. The position of the acetabular cup was referenced to the same planes, which were generated by the same points used during the operative procedure with the CAS. Thus, a true comparison of the inclination and anteversion given by the CAS and a CT scan could be made.
Accurate determination of the computer-generated planes is dependent on accurate points of registration. Wolf et al. , using a computerized kinematic model, reported substantial error in estimation of inclination and anteversion if points of reference to define the anterior pelvic plan were not accurate. Given this association, registration of the ASISs and pubic tubercles must be as accurate as possible. Spencer et al. , in a cadaver model, reported interobserver measurement variability for inclination and anteversion using an imageless CAS dependent on superficial landmark registration. The authors of that study imply overlying soft tissue obscures accurate registration of bony landmarks when obtained superficially. They speculate, in an obese patient with a large quantity of soft tissue, registration of landmarks may be even more inaccurate. Parratte and Argenson  reported an increase in acetabular cup position measurement error with an imageless CAS when the patients had a BMI greater than 27 kg/m2 compared with patients with a BMI less than 27 kg/m2. We found no correlation between BMI and accuracy in this study; however, the mean BMI in our patients was less than 27 kg/m2 (26.2 kg/m2). Less than 20% of our study cohort was obese, but the mean BMI of 26 kg/m2 is in line with BMI reported in hip arthroplasty studies. In a study of 800 primary THAs on 759 patients, the mean BMI preoperatively was 27.8 kg/m2 . Patel and Albrizio reported a mean BMI of 28.3 kg/m2 in 550 patients undergoing primary THA .
Our data compare favorably with results of others investigating the accuracy of CASs for acetabular cup placement (Table 3). Ybinger et al., in a recently published study using CT validation and involving four centers, reported an average difference of 3.5° ± 4.4° for inclination and 6.5° ± 7.3° for anteversion . They also reported inclination differences correlated with the thickness of the soft tissue overlying the ASISs, whereas the differences in anteversion showed a correlation with the thickness of the soft tissue overlying the pubic tubercles. These soft tissue thicknesses were measured on axial CT images. Dorr et al., in another recent study , reported a mean inclination difference of 4.4° and a mean anteversion difference of 4.1°. The mean BMI in their patients was 26.8 kg/m2 compared with the mean BMI of 26.2 kg/m2 in our patients. We found a smaller difference between CAS values and those measured on a postoperative CT scan for inclination and anteversion. This may be attributable in part to our practice of excluding patients with truncal obesity patterns from our study. Dorr et al.  used a method for registration of bony landmarks that punctured the skin for direct palpation of the bone with the probe. Despite this more direct method of registration, the precision and bias were less accurate than the values we obtained. The reason for our better accuracy most likely is the result of one or more of the following: differences in surgical technique, use of a different CAS, and/or use of a different method for CT analysis.
We believe an imageless CAS can provide accurate and precise real-time determination of the acetabular cup position in a patient population that does not have severe osteoporosis or substantial truncal obesity. The accuracy and precision values we obtained are among the highest reported to date for acetabular component position using an imageless CAS. Speculation remains regarding whether this level of accuracy can be achieved in patients in whom bony anatomic landmarks are difficult to palpate and identify. In such cases, the method of puncturing the skin for direct palpation, as reported by Dorr et al. , may provide sufficient accuracy.
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