Hip resurfacing has gained popularity as a bone-conserving alternative to total hip arthroplasty in the young and active adult with degenerative hip disease. In addition to proper patient selection, much of the success of the procedure relies on optimal implantation of the prosthesis1-3. Short-term failure of the resurfacing construct is often attributable to catastrophic femoral neck fracture4-6. Neck fracture frequently is the result of a poorly prepared femoral head in which femoral neck notching or varus implant alignment has occurred4-7. Poor preparation of the femoral head is potentially avoidable and relies on proper insertion of the initial femoral guidewire.
Commonly, manual mechanical alignment jigs are used to insert the initial guidewire into the femoral head. However, reliance on surgeon visualization for guidewire alignment and insertion position may lead to improper insertion of the guidewire and, ultimately, poor preparation of the femoral head8-11. Imageless computer navigation has shown promise for increasing the accuracy and precision with which the guidewire is inserted12-14 in comparison with conventional jig instrumentation8-11,15-18. However, the cost of current navigation systems presents a major limitation to their widespread use10.
Numerous manual alignment jigs are currently available to the surgeon, each employing different methods for alignment of the initial femoral guidewire. However, there is no clear indication as to which alignment method or jig characteristics provide the user with the opportunity for optimal guidewire placement. Furthermore, there appear to be no studies that have compared the accuracy and precision of the various guidewire alignment jigs with each other as well as with computer navigation. The purpose of the present investigation was to determine the accuracy and precision of the placement of the initial femoral guidewire with use of five different conventional alignment jigs and to compare these results with those of imageless computer navigation.
Five implant-specific, commercially available jigs were studied: two lateral pin jigs (jigs LP1 and LP2) (BHR [Birmingham Hip Resurfacing System; Smith and Nephew, Memphis, Tennessee] and MITCH [Stryker Canada, Hamilton, Ontario, Canada]), two neck centering jigs (jigs NC1 and NC2) (CONSERVE PLUS [Wright Medical Technology, Arlington, Tennessee] and Cormet [Corin Group, Gloucestershire, United Kingdom]), and one head planing jig (jig HP) (Durom; Zimmer, Freiburg, Germany) (see Appendix). The BHR, Cormet, and Durom devices are United States Food and Drug Administration-approved hip-resurfacing devices; the CONSERVE PLUS device is currently an investigational device; and the MITCH device is not available for use in the United States.
Four orthopaedic fellows (M.C., P.G., R.A.B., N.T.) participated in the study. None of the participating surgeons had extensive experience with any of the jigs or with navigation; however, all were familiar with hip resurfacing. Prior to the study, representatives from the manufacturers of the alignment jigs were invited to instruct the surgeons in the use of each jig. During the in-service demonstration, each surgeon used each jig to drill one guidewire into a synthetic foam femur. The surgeons received no further instruction for jig use, but they were permitted to consult the manufacturer's suggested surgical protocol for jig use during the study.
Conventional Guidewire Alignment Jigs
The two lateral pin jigs (jigs LP1 and LP2) each utilized a short pin that inserted into the lateral cortex of the femur. The position of the lateral pin was templated with use of digital anteroposterior unilateral hip radiographs and was defined as the vertical distance from the tip of the greater trochanter to the intersection of the lateral cortex and a line drawn along the center of the planned implant stem. The long arm of each jig was hooked onto the pin, and the cannulated rod was placed on the femoral head. A stylus was rotated around the femoral neck to check for notching before drilling the guidewire. One neck centering jig (jig NC1) had two arms that affixed directly to the femoral neck. Once the arms were fixed in place, the jig was visually aligned and a cannulated rod was tightened down onto the femoral head to drill the guidewire. The other neck centering jig (jig NC2) was fixed to the femoral neck with use of a circular clamp ring. A head centering jig was then attached to the neck clamp ring before drilling of the guidewire. A check to avoid notching was performed for both neck centering jigs once the initial guidewire was inserted. The head planing jig (jig HP) required an initial plane of the femoral head to position the base of the alignment jig. The centering jig was assembled to the impacted base, and the cannulated rod was used to align the guidewire. A stylus was used to detect notching before guidewire insertion.
Imageless Computer Navigation
The VectorVision SR 1.0 imageless computer navigation system (BrainLAB, Feldkirchen, Germany) was used in the study. The technique for patient registration and surgical planning has been described previously13. Briefly, the navigation system uses an infrared camera and reflective array system to register the patient's femoral anatomy intraoperatively. The computer produces a patient-specific morphed model on which planning of implant size and position can be carried out. Once a plan was accepted, the initial femoral guidewire was inserted into the femoral head with use of a navigated drill guide. The remainder of the standard surgical protocol was followed for preparation of the femoral head.
Synthetic Femora Study Design
Four surgeons used each jig three times to drill a guidewire in 10° of relative valgus alignment and neutral version into individual synthetic femora situated within a draped foam hip model (Pacific Research Laboratories, Vashon, Washington). The neck-shaft angle of the synthetic femora was 120°, with femoral neck anteversion of 8°. The order of jig use was randomized for each surgeon and for each guidewire insertion. Jigs were disassembled between each use. Each surgeon used imageless navigation to drill three consecutive guidewires into individual synthetic femora following the completion of jig use. Surgeons were timed for each use of the jigs and navigation. For the jigs, the guidewire insertion time was defined as the time to assemble each jig, to fix it to the femur, and to align and drill the femoral guidewire. For navigation, the guidewire insertion time was defined as the time to insert a Schanz pin into the lesser trochanter, to register the femur, to plan the position of the implant, and to drill and verify the position of the femoral guidewire.
Cadaver Femora Study Design
A single surgeon (M.C.) used each jig three times to align, but not to insert, the initial guidewire in 10° of relative valgus alignment and neutral version in each of ten dried femora. A single observer (M.O.) determined the native neck-shaft angle of each femur on an anteroposterior radiograph, and a second observer (M.C.) repeated the measurements. Each femur was registered with use of navigation before commencing jig use. The order of jig use was randomized between each guidewire alignment iteration; however, the head planing jig was used last as this jig required removal of femoral head bone, which would have altered the use of the other jigs. Jigs were disassembled between each use. Guidewire alignment for each use of a jig was verified with use of the navigation system. Following each use of a jig, the navigation drill guide was placed over the jig-aligned guidewire and then the computer verified the inclination and version of the guidewire. The surgeon was blinded to the verification result for each guidewire. Following jig use, a single guidewire was inserted into each cadaver femur with use of navigation.
Assessment of Guidewire Alignment
The coronal guidewire alignment angle was measured in a fashion similar to the standard stem-shaft angle assessment during hip resurfacing19 (Fig. 1-A). Guidewire version was assessed with use of a digital lateral radiograph in which the thinnest portion of the neck was projected onto the radiograph cassette. A line was drawn between the anterior and posterior cortices at the isthmus of the neck. Two parallel lines were drawn 5 mm medial and 5 mm lateral to the isthmus line. The midpoints of these two lines were connected and represented the native femoral neck axis. Version, or the implant stem-neck angle, was represented by the angle subtended by the guidewire and the femoral neck axis (Fig. 1-B). A single observer (M.O.) measured guidewire coronal and version alignment in the synthetic and cadaver femora. A second observer (M.C.) repeated the guidewire version alignment measurements performed on the cadaver femora to determine the repeatability of this measurement method.
Statistical Analysis
Alignment method accuracy was defined as the mean error from the planned relative 10° stem-shaft angle and neutral stem-neck angle as measured radiographically. Repeated-measures analysis of variance with Tukey post hoc analysis was performed to compare the differences in alignment error between alignment methods and between surgeons (for the synthetic femora) and between alignment methods and between femora (for the cadaver femora). Alignment method precision was defined as the average standard deviation across surgeons for synthetic femora and as the average standard deviation across femora for the cadaver femora. To evaluate jig precision, a Kruskal-Wallis analysis of ranks was used to compare average standard deviations for alignment method error, with post hoc Mann-Whitney U tests being used to determine differences between individual alignment methods. The time taken to use each alignment device was evaluated in a similar fashion. The level of significance was set at p < 0.05. An intraclass correlation coefficient was calculated to determine the level of repeatability between the two observers who measured both the native neck-shaft angle and guidewire version in the cadaver femora. A two-way random-effects model was used whereby both observer and rating effects were treated as random, with the observers considered as a random sample from a larger group of observers. Absolute, rather than consistency, measurement agreement was utilized in the statistical model because we believed that a single measurement of neck-shaft angle or version would determine these parameters clinically, rather than an average of measurements.
Source of Funding
No external funding was used directly in the completion of the manuscript. External funding was received by one of the authors (E.H.S.), but those funds were not used directly for the present study. However, funds for the study were obtained from an internal research fund for the acquisition of materials. Fellows donated their time, the various companies donated the jigs, and the salary of the lead author was covered by scholarship funding.
Synthetic Femora
The accuracy and precision of each alignment method are summarized in Table I. The only significant difference in coronal inclination accuracy occurred between the two neck centering jigs (jigs NC1 and NC2) (p = 0.028). With respect to version accuracy, one of the lateral pin jigs (jig LP1) produced a mean version error (and standard deviation) of 1.8° ± 5.9° of retroversion, whereas the other jigs and navigation erred in anteversion (range, 0.3° to 4.9°); the version accuracy of jig LP1 was significantly different from those for the head planing jig (jig HP), the other lateral pin jig (jig LP2), and navigation (p < 0.049 for all comparisons). Navigation had a range of error of 2° of varus to 2° of valgus with regard to coronal alignment and of 11° of anteversion to 5° of retroversion with regard to version. In comparison, the range of error for the jigs was two to eight times greater than that for navigation in terms of coronal inclination (Fig. 2-A) but was similar to that for navigation in terms of version (Fig. 2-B). There was a significant difference among surgeons in terms of coronal inclination accuracy (p < 0.011), but there was no difference among surgeons in terms of version accuracy (p > 0.156).
With regard to coronal inclination precision, navigation (mean, 1.3° ± 0.5°) was more precise than all of the alignment jigs (range of mean values, 1.3° to 7.2°). The precision of navigation was significantly different from that of jigs LP1 and HP (p < 0.026). Jig HP was the least precise alignment device (mean, 7.2° ± 2.3°), and this value was significantly different from those for jigs LP1 and LP2, jig NC2, and navigation (range of mean values, 1.3° to 3.2°; p < 0.021). There was no significant difference among the alignment method groups in terms of version precision (p = 0.299).
The mean guidewire insertion times for the alignment methods are shown in a figure in the Appendix. The guidewire insertion time for the head planing jig (jig HP) (mean, 14.1 ± 6.5 minutes) was significantly longer than those for the rest of the jigs and for navigation (p < 0.001). The guidewire insertion time was shortest for the neck centering jigs (jigs NC1 and NC2), with the lateral pin jigs (jigs LP1 and LP2) (p < 0.006) and navigation (p < 0.001) having significantly longer insertion times. With the numbers available, there were no differences between the two lateral pin jigs (p = 0.160) or between the two neck centering jigs (p = 0.887) in terms of guidewire insertion time. We did not identify a significant difference in guidewire insertion time among the surgeons in the study (p = 0.913).
Cadaver Femora
There were significant differences between the two lateral pin jigs (p = 0.027) and between the two neck centering jigs (p < 0.001) in terms of coronal inclination accuracy (Table I). Jig NC1 tended to place guidewires in a valgus orientation compared with plan (mean, 2.1° ± 3.1°), and this value was significantly different from those for all other jigs (range, 1.9° of varus to 0.3° of valgus, p < 0.049). The analysis of version accuracy showed that jig LP1 produced a mean version error of 1.5° ± 3.2° of retroversion; this value was significantly different from the other jigs, which tended to err in anteversion (range, 0.4° to 1.8°; p < 0.010). Navigation had a range of error of 1° of varus to 2° of valgus with regard to coronal alignment and of 4° of anteversion to 4° of retroversion with regard to version. The range of error for the jigs was three to seven times greater than that for navigation in terms of coronal inclination (Fig. 3-A) but was similar to that for navigation in terms of version (Fig. 3-B). Coronal inclination accuracy was not dependent on femoral specimen (p > 0.377); however, version accuracy was, as there were multiple differences in accuracy among specimens (p < 0.030). In particular, one femoral specimen was associated with a version error of 5.5° of anteversion, which was significantly different from the values for all other femoral specimens (p < 0.002). The mean difference between the two observers was 1.1° ± 2.3° with regard to coronal neck-shaft angle measurements and 1.4° ± 1.9° for the guidewire version measurements, with intraclass correlation coefficients of 0.85 and 0.70, respectively.
Jig HP demonstrated significantly less precision than all other jigs with regard to coronal inclination (4.7° ± 1.8°; p < 0.023) (Table I). Similar to the synthetic femur portion of the study, no significant difference in version precision was observed among the jigs (p = 0.120).
The present study demonstrated that imageless computer navigation was superior to the use of conventional jigs in terms of coronal inclination accuracy and precision but fared no better with regard to version. These findings support previous work comparing coronal femoral guidewire placement by conventional means and imageless computer navigation. Davis et al., in a cadaver study, investigated coronal guidewire alignment accuracy with use of a conventional lateral pin jig and compared it with imageless navigation15. They found that the lateral pin jig tended to place guidewires in varus alignment relative to the planned guidewire angle and that the range of error in placement of the initial guidewire was reduced from 15° in the jig group to 8° in the navigation group. In a clinical retrospective cohort study, Resubal and Morgan used imageless navigation to achieve a stem-shaft angle accuracy of ±5° of plan in forty-five of forty-five cases while achieving this accuracy in only 76% of 131 cases with use of a conventional lateral pin jig10. Comparable results were demonstrated in a similar retrospective clinical review in which an accuracy of ±5° of plan was achieved in all fifty-one procedures performed with computer navigation but in only 62% of eighty-eight procedures performed without computer navigation8.
While numerous studies have demonstrated the increased accuracy and precision of imageless navigation compared with conventional jigs in terms of coronal implant alignment, only a few studies have investigated femoral component version. In a study involving dry bone models, Cobb et al. evaluated guidewire version associated with the use of (1) imageless navigation, (2) conventional instruments, and (3) computed tomography-based navigation20. All three methods showed comparable accuracy; however, imageless navigation had significantly poorer precision (standard deviation, 9°) compared with computed tomography-based navigation (standard deviation, 2°; p < 0.001) and a conventional neck centering jig (standard deviation, 5°; p = 0.015). A similar finding was reported in a cadaver study in which imageless navigation was less precise (standard deviation, 4.4°) than a conventional head planing jig (standard deviation, 3.2°) in terms of version; however, this difference was not significant (p = 0.20)16. In contrast, imageless navigation was shown to be significantly more precise in terms of femoral component version (standard deviation, 1.53°) than a mechanical lateral pin jig (standard deviation, 4.36°; p = 0.025) whereas no difference in accuracy was demonstrated between the two methods10. The current work supports the finding that imageless computer navigation provides results similar to conventional jigs in terms of guidewire version accuracy while also providing a comparable level of precision. Our results with use of cadaver femora showed that proximal femoral morphology may have an impact on the visualization of version; however, no such effect was detected for coronal alignment.
Next to navigation, one of the lateral pin jigs (jig LP1) provided the most accurate coronal alignment of the initial guidewire. This may have been the result of preoperative measurement of the lateral cortical pin-insertion point on the anteroposterior radiograph, thus providing a reference position to aid in coronal alignment. However, radiographic magnification or scaling variability could lead to erroneous lateral pin-insertion measurements, which ultimately may impact the use of the jig and insertion of the initial guidewire. One of the neck centering jigs (jig NC2) provided the most precise coronal guidewire alignment next to that of navigation. The circular clamping device provided repeatable placement of the head-centering jig but was limited in terms of the degree of valgus that could be introduced on an oval-shaped femoral neck. Consequently, all guidewires in the study were inserted in varus with respect to the planned stem-shaft angle of 10° of relative valgus. The initial medial head plane appeared to have a considerable impact on the accuracy and precision of guidewire insertion with the use of the head planing jig. If the desired entry point could not be obtained, the base of the jig required repositioning, which often necessitated replaning of the femoral head. This tended to increase the time taken to insert the femoral guidewire and may have resulted in the larger ranges of error in terms of the accuracy and precision of guidewire insertion. The tendency for longer guidewire insertion times agrees with the findings of similar studies comparing a head planing jig and imageless navigation16,17.
The findings of the current study are limited to the devices that were examined and cannot be extended to similar devices without further investigation. Guidewire insertion with use of synthetic and cadaver femora situated within a foam hip model may not exactly replicate the clinical setting for hip resurfacing and is a limitation of the current study. The test conditions aimed to present an optimal setting for evaluating the accuracy and reliability of conventional jig instrumentation and imageless navigation in which complicating factors such as proximal femoral deformity and soft-tissue effects were removed. Clinically, we anticipate lower levels of accuracy and precision and larger ranges of error caused by factors such as variable patient anatomy and the presence of proximal femoral soft tissue. In addition, a potential bias existed for the surgeon who performed the guidewire alignment in the cadaver portion of the study. The surgeon had already utilized the jigs on synthetic femora and as a result may have become more proficient in their use. Furthermore, we did not perform an intraobserver reliability analysis to identify the level of repeatability of jig use for each surgeon. Last, while the intraclass correlation coefficient between the two observers was acceptable for the measurement of the neck-shaft angle in the cadaver femora, it was marginally lower than that considered acceptable for the measurement of guidewire version21. Further investigation is required to establish the optimal method of assessing guidewire version by means of lateral radiographs in hip resurfacing.
Correct alignment of the initial femoral guidewire is vital in order to prepare the femoral head properly for hip resurfacing. The choice of conventional alignment device may influence the accuracy and precision of guidewire placement, ultimately impacting femoral component implantation. Imageless computer navigation can provide accurate and precise coronal alignment of the initial femoral guidewire, superior to that of conventional instrumentation, but performs similarly to conventional jigs with regard to femoral guidewire version.