Restoration of the hip center is considered important for a successful THA and involves achieving the correct limb length, anteversion, and offset. Limb-length inequality is a particular problem in THA [8, 11, 16, 17]. Proper cup position and total anteversion are important for ROM and to decrease impingement and wear [3, 6, 12, 14, 19, 20]. Appropriate offset is crucial for soft tissue balance, restoration of the center of rotation of the hip, and decreasing impingement [4, 10, 14]. Anatomic gender differences may further confound the surgeons’ ability to recreate the head center in certain cases . Preoperative planning is essential for restoration of the femoral head center and correct hip biomechanics [21, 23].
Modular femoral neck designs can make a complex operation easier by supplying more options and offering more flexibility. Systems offering this added modularity allow the surgeon to adjust offset, version, and limb length and to make adjustments to these parameters independently of one another during preoperative planning and in the operating room. However, some studies report risks associated with added modularity, including implant fractures, fretting, and third-body wear for modular necks [1, 24-27]. In light of these issues, the use of a modular neck stem in THA remains controversial.
We therefore asked (1) whether use of a stem with a modular neck would restore limb length and offset more accurately than a stem with a nonmodular neck, and (2) whether patients who received modular neck systems had better hip scores or a lower frequency of complications and reoperations than those receiving nonmodular stems.
Materials and Methods
This was a retrospective observational study using a prospectively maintained database to evaluate two cohorts of patients. One cohort received a nonmodular neck (Zimmer® M/L Taper; Warsaw, IN, USA) and the other cohort received a modular neck (Zimmer® M/L Taper Kinectiv). Both stems are identical except the modular neck stem has a separate neck piece that allows for 60 different options for head center versus 10 options with the nonmodular neck. In all cases, the same acetabular components were used, which consisted of a Trilogy® fiber metal-backed cup and metal-on-polyethylene bearing surface with highly cross-linked polyethylene (Zimmer) [9, 22]. No elevated liners were used. The study was approved by the institutional review board.
From August 1, 2005 to December 31, 2009, 883 patients underwent THA by one surgeon (PJD) and consented to be included in a prospective followup database. Five patients who had bilateral THAs were excluded. Of the 878 patients who had a single arthroplasty, 284 received a hip stem with a nonmodular neck and 594 received a hip stem with a modular neck. Initially, we used nonmodular neck Zimmer® M/L Taper stems exclusively. On introduction of the otherwise identical modular neck stem (May 9, 2007) and up to December 31, 2009, we switched to the modular neck almost exclusively, with only 10 nonmodular neck cases performed during this period. Of these 10 cases, which were included in the analysis for this study, seven were the result of increased offset issues (this is a drawback of the modular neck stem in that it offers less offset than the nonmodular neck), and the other three were because of poor bone density with a Dorr Type C pattern, for which a fit-and-fill stem was used. These decisions were made during preoperative radiographic templating or planning.
Patient demographics and clinical profiles are similar between the two cohorts, except for diabetes and diagnosis (Table 1).
Complete followup at a minimum of 2 years (mean, 2.4 years) was available for 197 patients in the nonmodular group (69%) and 459 patients in the modular stem group (77%). Outcomes at 1 and 2 years were similar for matched patients. This finding allowed us to merge the completed outcomes surveys from 1- and 2-year followups into a new group where followup was completed at either 1 or 2 years. This combination resulted in 90% completion of followup. For unexplained reasons, there was differential loss to followup between the modular (93%) and nonmodular (84%) groups (p < 0.001).
Study Outcome End Points
End points for this study include clinical and radiographic measurements and revisions and complications.
Clinical end points include apparent or clinical limb-length evaluation and Harris hip and SF-12 scores. Before the operation, all patients had an evaluation of apparent limb length. With the patient in the lateral decubitus position, an osteotome was placed at the level of the patellar tendon and the operative leg was compared with the nonoperative leg. Preoperatively patients whose apparent limb length was short were asked to stand on blocks of varying sizes to determine the desired correction. This became the target limb length to reproduce postoperatively, as described subsequently in the surgical technique. Clinical reevaluation of limb length was repeated postoperatively and at 1-year and 2-year followups, and Harris hip and SF-12 scores were assessed preoperatively and at 1- and 2-year followups.
Radiographic end points include leg length and offset, evaluated before and after surgery, in a subset of patients who had no contralateral deformity or prior THA and whose radiographs were available electronically. Offset was based on the contralateral hip. Being within 5 mm of offset difference is considered a good result and tends to improve stability and hip biomechanics. For the purposes of this study, we compared the mean offset difference and the percentage of hips with an offset difference of 5 mm or less between the two study cohorts. We also calculated the percentage of hips in each stem group that was of equal radiographic leg length (within 5 mm).
The desired head center was determined preoperatively by templating the head center on the acetabular radiograph. The films were either templates from acetate templates (early cases) or digital radiographs (later cases). The acetabular head center was templated first and then the femoral stem size was matched to the acetabular head center. The distance above the trochanter was measured to achieve the correct cut to recreate this offset and leg length. A radiographic marker was used in all cases to adjust for magnification and all radiographs were obtained from a standard distance. Version was determined intraoperatively. A mini-posterior approach was used in all cases in this series. In both cohorts, the femur was prepared first to best determine the optimal femoral anteversion. The acetabular cup then was placed based on the femoral anteversion so that a combined version of a 30° to 50° target zone could be attained based on the Ranawat sign . If a cup size of 50 to 56 mm was achieved, then a 36-mm femoral head was used. A 32-mm femoral head was used in cups up to 50 mm. A 40-mm femoral head was used for cup sizes of 58 mm or larger. Components found to impinge or dislocate during a trial were changed or the cause of impingement (such as osteophytes or soft tissue) were removed. Anteverted or retroverted necks were not used to correct component malpositioning. Reduced or extended offset necks were used when outlier offset, based on preoperative radiographic templating, was encountered. The goal was to recreate the anatomic head center and provide for a clinically stable THA. For patients with a long leg resulting from pelvic obliquity such as scoliosis, the limb length was not changed. For patients with an apparent short leg, the limb length was corrected to the target length determined by the block test in the clinic.
The active critical hip pathway was used for post operative care in this series [2, 7, 13].
Continuous variables are presented as mean ±SD and comparisons between the two groups were done by unpaired t-tests. Categorical variables are presented as percentages and the comparisons were done by chi-square or Fisher's exact tests. Hip function and quality-of-life measures are summarized as mean ± SD and compared by a linear mixed effect model for repeated measures. Freedom from revision was computed by the Kaplan-Meier method and compared by the log-rank test. Statistical analysis was performed using PASW 17 (SPSS Inc, Chicago, IL, USA) and R 2.15 (http://www.R-project.org).
In the modular neck cohort, a greater proportion of patients had equal (within 5 mm) radiographic limb length (89%, compared with 77% in the nonmodular cohort, p = 0.036), and a smaller offset difference was observed (6.1 versus 7.5 mm, p = 0.047) (Table 2). However, the proportions of patients having an offset difference within 5 mm were similar.
The slightly enhanced accuracy of the modular neck in restoring the head center did not translate into better short-term (1-2 years) outcome scores, revisions, or complications. A smaller proportion of patients in the modular neck cohort achieved equal apparent or clinical limb length at 1 year (85% versus 95%, p < 0.001) and at 2 years (81% versus 94%, p < 0.001). Harris hip and SF-12 scores were not different between the groups (Table 3), and modularity did not affect these measures, even after adjusting for age, sex, diabetes, diagnosis, and head size (Table 4). There were no differences in revision rates at 2 years. Freedom from revision at 2 years was similar between the two groups, at 99% (97%-100%) and 99% (98%-100%) for the nonmodular and modular neck groups, respectively (p = 0.94). There were 10 revisions, three in the nonmodular neck group (two for dislocations and one for fracture of the femur) and seven in the modular neck group (two for dislocations, one for fracture of the femur, two for deep infections, one for fracture of the greater trochanter, and one for other). Finally, there were no differences in complication rates between the two groups (Table 5). No other safety issues (such as trunnion fractures or neck dissociations) were encountered with the modular neck during the period of this study.
The purpose of using a modular implant is to improve the head center by allowing for independent leg length, offset and version, thus improving hip biomechanics. We evaluated clinical and radiographic differences between modular and nonmodular hip stems. Although modularity is widely used, its superiority in outcome scores, hip biomechanics, and complications have not been shown over those of conventional implants. We found that use of modular neck stems did not improve hip scores or reduce the likelihood of complications or reoperation. Because of their reported higher risks [1, 5, 24-27], there is no clear indication for modularity with a primary THA, unless the hip center cannot be achieved with a nonmodular stem, which is extremely rare.
One weakness of this study is that patient allocation was not randomized between the two treatment groups. Essentially, we had two consecutive cohorts spanning 5 years with nonmodular neck cases from the earlier years and modular neck cases from the later years. Possible differences in technology, improvements to surgical technique, and other changes that are hard to characterize in the context of a retrospective study potentially could confound the results of this study. These differences would tend to favor the later group, making the small observed differences that favored the modular stems even less likely to be clinically important. This retrospective observational study was not powered to detect differences in uncommon complications, although the point estimates suggest that the likelihood of important differences is low, and if a difference cannot be detected in a study of nearly 900 THAs, it seems unlikely to be clinically important. In addition, accurate parameter assessment is difficult. For instance, limb-length estimation can be different between the clinical (apparent) and radiographic (true) assessments. This was a difficult aspect of this study and a shortcoming in that limb length can vary between the two measurement methods and with time in the same patient owing to multiple factors such as abduction contractures, pelvic obliquity, and scoliosis. Furthermore, patients’ perceptions of limb length usually come from the apparent limb length, which also can change with time. Using the latest scanograms or CT technology to improve the true limb-length measurement is not justified in this setting and probably would not eliminate all of the discrepancies between the two assessment methods in any case. The intangible variables of offset and version are even harder to accurately reproduce clinically over radiographic parameters. Such intangibles include stability, soft tissue laxity, presence of osteophytes, congenital deformity, impingement issues, and surgeon preference. Because radiographs were available for only a subset of patients, the radiographic results could be confounded by selection bias.
In our short-term study, we showed that use of a modular neck improved the surgeon's ability to achieve correct offset based on radiographic parameters. However, the slight improvement in offset difference with a modular neck was small and the clinical significance questionable. Most patients in either the modular or nonmodular cohorts were outside the 5 mm offset difference target. One could argue that the addition of more modular options does not necessarily make a procedure easier, since the criteria for using these options may not be as clearly documented. One of the learning curves we overcame was to evaluate limb length first, which seemed to be easiest intraoperatively, and then determine offset and version. Although there are some situations that seem to call for unusual stem designs, perhaps including modularity, we find these situations rare and the majority of patients can be treated with nonmodular neck stems. In terms of limb-length equality, we found disparate results. The radiographic limb length was closer in the modular neck group, but the clinical (apparent) limb length, which we believe to be the more important parameter, favored the nonmodular neck group.
In addition, none of the observed differences in terms of radiographic leg length or offset were associated with improvements in outcomes scores, with a reduction in the frequency of important complications, or with a reduction in the frequency of revision surgery. Although we saw no complications associated with modularity, our followup was short term. In addition, there have been studies showing that corrosion is the main concern with these taper junctions [1, 5, 24-27]. The soft tissue reactions and destruction seen with the abductor mechanisms in some of these reported cases of trunionosis is compelling. We have identified five cases of corrosion which either happened outside our study period or did not occur in patients who consented to inclusion in our registry. We felt these complications were significant and should be reported. All these cases occurred at the head-neck junction in 36-mm femoral heads. Three involved the modular stem and two involved the nonmodular stem. Among the three modular cases, one used a standard offset neck and the other two used extended necks (one straight and one anteverted). The nonmodular cases used extended necks. The modular stems were revised to a new head-neck junction and ceramic head. The nonmodular stems were revised to a ceramic head with a titanium sleeve. There were significant soft tissue reactions in two of these cases and débridement was performed to remove the metallic stained tissue and to stop the inflammatory response. Fretting corrosion is known to be a time-dependent process. Therefore, in this short-term series, this type of complication might not have had a chance to reveal itself, particularly in the modular neck group, which is the more recent of the two nearly sequential cohorts. Even though this modular design has a cobalt-chromium-titanium head-neck junction, the titanium-titanium neck-stem junction still could pose risk for corrosion. For these reasons the senior author (PJD) now almost exclusively uses nonmodular stems and ceramic femoral heads to decrease the possibility of corrosion. Of our 450 recent consecutive THAs, the modular stem was used only once in a slender female. The patient in this case had a modular stem inserted on the opposite side in an extremely varus hip to avoid excessively lengthening the leg.
Although there were no fractures or stem failures in this large and relatively unselected series, modular neck stems did not lead to improved clinical hip scores, reduction in complications, or decreased likelihood of revision hip surgery, and there are known risks of modularity (pseudotumors, implant fractures, fretting, third-body wear, and trunnion corrosion) [1, 5, 24-27]. Therefore we currently would consider only modularity if the clinical hip center is anticipated to be an extreme outlier (ie, the clinical hip center is anticipated to be greater than 5 mm away from the desired target, resulting in a significant leg-length discrepancy or offset issue). Long-term followup is needed to assess the ultimate durability of modular implants and to determine the frequency of complications associated with modular devices.
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