The number of THAs performed in the United States is projected to increase over the next few decades, and revision procedures are also expected to concomitantly rise . The most common reasons for revision surgery are aseptic loosening, hip instability, periprosthetic osteolysis, and infection [20,21]. Acetabular bone loss in patients undergoing revision THA is not uncommon and often constitutes a considerable challenge to the surgeon. Some of the potential treatments for acetabular bone defects include smaller hemispheric cups with augments, utilizing impaction grafting, structural allograft reconstruction, modular porous metal augments, ring and cage reconstruction [6,7,9–11,14,22], oblong cup reconstruction , and jumbo cups [3,5].
In many cases, to achieve better fixation in the remaining bone, larger diameter acetabular components (jumbo cups) and superior reaming may be necessary [3,5]. However, this is likely to elevate the hip center of rotation (COR), which may lead to altered biomechanics, specifically, insufficiency of the abductor muscles, altered gait, and increased risk of dislocation from impingement [8,16,17]. It has been shown that in patients who have dislocated, the prosthetic COR was often located superolateral or inferolateral to the native COR at distances > 5 mm away [1,3,11]. More recently, a new acetabular shell has been designed to more closely replicate the native hip COR while maintaining adequate fixation. This novel hemispheric acetabular cup integrates an eccentric COR by virtue of a thicker superior rim with peripheral screw holes directed into the posterior column of the pelvis in addition to dome screws [5,15]. The implantation of this component does not require specialized instruments or surgical techniques.
The purpose of this study was to compare the outcomes of this newer eccentric COR acetabular cup design with conventional hemispheric cups in revision THA. Specifically, we assessed the differences in (1) vertical COR displacement and (2) horizontal COR displacement between the two groups.
Patients and Methods
Between January 2016 and April 2016, five reconstructive surgeons at five institutions utilized a newer highly porous acetabular shell, the Restoration® Anatomic Shell (Stryker Orthopaedics, Mahwah, NJ, USA), designed with peripheral screw holes and eccentric COR to allow for restoration of center of hip rotation in revision THA (Fig. 1). We included all patients in the analysis who received this device. During this time, the general indications at these sites for using the new device included Paprosky Stage IIA, IIB, IIC, or IIIA acetabular defects. This yielded 29 patients. Those patients (n = 5, not included in analysis) who had Stage IIIB defects or had severe defects of the superolateral acetabulum and the posterior column received triflange cups or acetabular cages with hemispheric cups. The mean age of the patients in the new acetabular cup group was 61 years (range, 25-74 years), and the group contained 15 men and 14 women. All revision THAs were performed by five fellowship-trained adult reconstructive surgeons (DH-F, ALM, SFH, MAM, and MDR–who was the designer of the cup).
These patients were matched in a one-to-two ratio by cup size and sex to a cohort who underwent revision THA with conventional hemispheric cups between January 2015 and May 2016. This matching cohort was comprised of 58 patients (27 men, 31 women) who had a mean age of 60 years (range, 19-77 years) (Table 1). There were no differences in demographics between the study and control groups (Table 1). There was no difference in cup diameter between the two groups (63 ± 6 mm versus 61 ± 4 mm; p = 0.214). There were no apparent differences in the severity of bone loss before the revision in both cohorts, as evidenced by Paprosky staging of preoperative radiographs (Table 1) .
To determine hip COR, radiographic measurements were performed by a single observer who did not perform the procedures (AK) using standing AP pelvic radiographs from the patient’s initial postoperative visits (typically between 2 and 6 weeks). No patients were excluded as a result of poor quality of the radiographs. The magnification conversion factor was calculated using the known diameter of acetabular cup. An interteardrop line was drawn connecting the most inferior borders of the two pelvic teardrops, which served as a baseline for all other measurements. For the native hips, a circle contiguous to the acetabulum was drawn and the center of the circle was determined to represent the hip COR (Fig. 2) . For prosthetic hips, a circle contiguous to the external border of the acetabular cup was used to find the COR. The prosthetic femoral head COR was not used as the marker for COR because it was not always visualized on the postoperative AP radiographs as a result of opacity of the acetabular cup. The vertical displacement in the COR of the new acetabular implant was adjusted by using measurement parameters for each cup size provided by the manufacturer (Table 2). All vertical measurements were made from the interteardrop line for both the revised and native hips. All horizontal measurements were made from a line perpendicular to the interteardrop line, which passed through the center of the teardrop (Fig. 2). There were four patients (14%) in the new cup group and seven (12%) patients in the control group who had contralateral THAs. All measurements for contralateral acetabular cups were performed using the same methodology as for the side of interest.
The vertical hip COR displacement was calculated by subtracting the distance (in millimeters) between the native hip COR to the interteardrop line from the distance between the contralateral prosthetic hip COR to the interteardrop line. For the new cup COR calculations, the manufacturer’s inferior displacement distance was subtracted from the hip COR displacement. The horizontal hip COR displacement was calculated similarly using the center of the teardrop (line perpendicular to the interteardrop line) as a baseline. The proportion of patients who had hip COR elevation within 20 mm was calculated. Comparisons were performed using chi-square tests for categorical and t-tests for continuous variables. A p value of < 0.05 was set as the threshold to determine statistical significance for the results. All statistical analyses were completed using SPSS Version 21 (IBM Corp, Armonk, NY, USA).
There was a smaller mean vertical COR displacement in patients who had the novel offset COR cup (3.5 mm; range, -12 to 15 mm; mean difference, -7.3; 95% confidence interval [CI], -13.2 to -1.5) when compared with those who had the conventional cup (10.5 mm; range, -4 to 50 mm; mean difference, -7.3; 95% CI, -12.5 to -2.2; p = 0.003) (Table 3; Figs. 3, 4). There was a higher proportion of patients in the new cup group who had total vertical displacement < 20 mm (29 of 29 in the new cup group [100%] versus 50 of 58 in the control group [86%]; odds ratio, 0.86; CI, 0.5-1.6; p = 0.036).
There was no difference in horizontal COR displacement between the patients who did and did not receive the novel cup (-0.06 ± 6.1 versus 1.7 ± 7.1; mean difference, -1.8; p = 0.903) (Table 3).
The incidence of revision THA in the United States is projected to increase over the next several decades . Patients undergoing revision are likely to present with acetabular bone loss, which may result in more complex reconstructions. Often, larger diameter acetabular cups are utilized and the acetabulum may be reamed superiorly, which can potentially lead to a higher center of hip rotation. In addition, as the cup diameter increases, the COR shifts superiorly. This can potentially lead to poor postoperative hip biomechanics, cause a leg length discrepancy, and result in poor clinical outcomes from insufficiency of the abductor muscles, altered gait, and increased risk of dislocation from impingement [4,8,16,17,19]. Therefore, a new revision acetabular shell was developed in an attempt to more closely replicate the native COR while still allowing standard implantation techniques. The implantation of this novel acetabular cup in revision THA resulted in less vertical displacement of the COR of the hip when compared with a conventional hemispheric shell. The novel cup did not influence the horizontal displacement of the COR. Although hip COR was improved with the use of this novel acetabular design, with potential improvement in hip biomechanics, more studies must be performed to demonstrate durable fixation and improved clinical outcomes before its widespread adoption.
There were several limitations of this study. Patients were not randomized to receive this device; therefore, it is possible that the surgeons could potentially select easier cases to test the new device, which could potentially lead to better outcomes in the test group. The control group was not matched to the study group by the degree of acetabular deficiency; however, the cohorts were matched by cup size. Consequently, there were no apparent differences in the severity of bone loss before the revision in both cohorts, as evidenced by Paprosky staging of preoperative radiographs. The small sample size and use of this cup by five fellowship-trained specialists in hip reconstruction could theoretically compromise generalizability. In addition, the radiographic review was performed by a single observer; therefore, inter- and intraobserver variability was not assessed. Only vertical and horizontal displacement was considered and only radiographic appearance was assessed in this analysis, and therefore, the effect on biomechanics, implant fixation, and longer term clinical outcomes is not really known. In addition, no cup orientation analysis was presented and no lateral radiographs were evaluated to allow assessment of spatial orientation of the cup.
The mean COR elevation of 10.5 mm in the hemispheric cup group was very similar to a previous radiographic study performed by Nwankwo and Ries (one of the senior authors of this article)  of 98 jumbo cup revisions, which demonstrated an average head center elevation of 11 mm. Faizan et al.  performed an in vitro three-dimensional computer simulation study evaluating hip COR and screw trajectory improvements of this novel offset COR acetabular cup (Table 4). A computer simulation program, Stryker Orthopedic Modeling and Analytics (SOMA) (Stryker Orthopaedics), which integrates 265 pelvic CT scans of skeletally mature patients, was used to study the simulated revision THA with various reamer diameters and shell sizes. For each diameter increase, the hip COR displacement, screw fixation options, and screw fixation engagement were measured for both conventional “jumbo” cups and the new offset COR cup. The use of the new offset COR cup, when compared with the “jumbo” cup, yielded smaller shifts in COR of the hip for both smaller (+6 to +18 mm) amounts of overreaming (2.1 versus 2.2-5.3 mm) and larger (+20 to +30 mm) amounts of overreaming (4 versus 8.8 mm). In addition, there were 25% to 300% more screw fixation options (number of viable screw holes) for the novel offset COR cup design and 34% to 139% increase in screw engagement (additive length of screw contact with the pelvic bone, excluding contact of < 11 mm and > 60 mm to account for available screw sizes on the market) for shell sizes 60, 68, and 74 mm. Of note, the screw engagement was 22% lower in the new cup group for 56 mm diameter.
A recent retrospective study performed by one of the senior authors (MAM)  assessed the relationship between the COR displacement and prosthetic hip dislocation of 34 revision THAs resulting from nontraumatic, implant-related prosthetic hip dislocation at one institution (Table 4). The authors radiographically measured leg length discrepancies, angles of acetabular inclination and anteversion as well as COR displacement. Twenty-eight of the patients had leg length discrepancies (mean difference, 7 mm; range, 0-26 mm). The majority of the patients (70%) had their inclination (mean, 43°; range, 23°-68°) and anteversion (mean, 12°; range, 2°-30°) angles within Lewinnek’s safe zone . The results of this study demonstrated that all patients had prosthetic COR displaced supero- or inferolaterally. Eighty percent of patients had their acetabular cups placed at a distance > 5 mm from native COR. In a study of 60 hips, Asayame et al.  found a negative correlation between abductor muscle strength and height of hip center to pelvic height ratio (r = -0.568; p = 0.001), which suggests that inferomedial position of the hip COR maximizes postoperative abductor function after THA.
Although hip COR was improved with the use of this novel acetabular design, with potential improvement in hip biomechanics, we recommend more studies be performed to determine that it achieves durable fixation and improved clinical outcomes before its widespread adoption. The costs and risks associated with new implant designs must still be justified by studies that evaluate implant durability and patient-reported outcome scores, which were beyond the scope of this preliminary report.
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