Femoral Neck Cut Level Affects Positioning of Modular Short-Stem Implant

Abstract

A trend in total hip arthroplasty surgery has been to design more bone-preserving procedures, especially for younger patients. This study investigated the final implant positioning of a short metaphyseal femoral neck type of implant to determine whether leg length, caput collum diaphysis (CCD) angle, and offset could be re-created with different levels of femoral neck resection. Ten cadaveric hips in 6 whole-body specimens were used, with 3 fiducial markers to allow registration of computer navigation points to computed tomography scan data. Three femoral neck resection levels were investigated: 0 mm, +5 mm (the recommended level of resection), and +10 mm from the base of the femoral neck. Results showed that the CCD angle was significantly higher with 0-mm neck cut and the offset was lower, whereas the highest neck cut had longer leg-length results. Surgeons who use a short metaphyseal stem need to realize the importance of a proper femoral neck cut to restore anatomic parameters as well as the possible benefit of computer-assisted surgery to restore these anatomic parameters during surgery.

The number of primary total hip arthroplasty (THA) procedures that are performed continues to grow exponentially, and the trend is expected to continue in the United States over the next decade as Baby Boomers continue to age.¹ Consequently, THAs will continue to be one of the important cost factors for health care.^2,3 Furthermore, THAs are being increasingly performed in younger patients as well.^2,4,5

Since the first THA was performed by Sir John Charnley, many improvements in implant design and materials have been developed. These innovations have influenced several trends for THA. The current trends over the past few years have been (1) minimally invasive surgery,⁶ (2) bone-preserving procedures,⁷ (3) increasing modularity, and (4) computer-navigated techniques.⁸

Minimally invasive surgery reduces damage to periarticular muscles, allowing for faster recovery and less pain.^9,10 Bone-preserving procedures include hip resurfacing¹¹ and now shorter metaphyseal stems.^12-14 The impetus for this trend has been the growing number of younger patients who are now benefiting from the procedure. If the femoral implant fails in 5 to 15 years, enough femoral bone stock should remain for conversion to a standard proximal third coated femoral primary stem. Modular designs are also important for the younger hip osteoarthritis patient when developmental hip dysplasia, Perthes disease, or other anatomic variant is the cause of the early degeneration.^4,5 Computer navigation techniques have come into favor for many surgeons, especially for minimally invasive surgery for which direct visualization is not always appropriate to assess component malposition or proper leg length and offset.^15-17

To facilitate these trends, orthopedic companies have undertaken the development of new implant concepts that are different from the well-known standard straight stem. Among these new concepts are resurfacing and metaphyseal short-stem implants.^11,13 These new products preserve more bone than a straight stem, and they use a different fixation mechanism and are implanted more proximally than a straight stem.¹⁴

Metha, a modular metaphyseal short hip stem (B. Braun Aesculap, Tuttlingen, Germany), was brought to the market in 2004 and has since been implanted in more than 17,000 patients, showing good short-term clinical results in several studies.^18,19 Unlike many other stems, this design is not a “shortened” version of a straight stem. It is a true metaphyseal stem based on previous short-stem designs.²⁰ Modularity offers a large number of combinations to restore the joint anatomy. However, a realistic estimation of the true implant position and therefore the best choice of the implant components could be demanding, especially without use of computer navigation.

The majority of previous studies estimating the implant position with respect to the bone and the capability of the Metha hip design to restore the individual anatomic conditions were performed using standard radiographs.²¹ In this cadaveric study we used computer navigation (OrthoPilot, B. Braun Aesculap) and computed tomography (CT) scans with fiducial markers to answer whether the level of the neck cut on the femur has a significant effect on positioning of the metaphyseal type stem and whether the design of the Metha stem allows for restoration of the individual anatomy.

Materials and Methods

In this study, 10 hips in 6 fresh-frozen human cadavers (3 male; 3 female) were used. In 2 cadavers (1 male, 1 female), only the left leg was available. Three plastic fiducial screws were placed in every femoral bone of the thawed cadavers to register computer navigation data to the CT scan. Screws were placed in each femur at the medial and lateral epicondyle and the base of the greater trochanter. A small conical groove in each screw head fit to the tip of the registration probe. After the fiducial markers were placed, the cadavers underwent a spiral CT scan providing 0.5-mm-thick slices. The CT data were segmented using Amira software (Version 5.0; Visage Imaging, Inc, Andover, Massachusetts). During the segmentation, the outer shape of the bone as well as the inner femoral canal were found. The following structures and their coordinates were identified: fiducial points at the bottom of the grooves in the screw heads, proximal anatomic femoral axis, center of the femoral head, and femoral neck axis.

Using a standard posterior approach, Metha rasp implantations were performed by a senior fellowship-trained surgeon (W.M.M.). This step was performed for 3 different resection levels in each cadaver. The lowest point of the upper femoral neck surface was used to define the zero resection level. This is often referred to as the shoulder of the femoral neck (Figure 1).

Figure 2: Example of the re-created model in CATIA

Figure 1: The lowest point of the lateral base of the femoral neck is referred to as the shoulder. This is the point from which the resection level of the femoral neck was measured for this study. The recommended resection level for the implant was 5 mm above this point. Figure 2: Example of the re-created model in CATIA (Version 5; Dassault Systemes, Suresnes Cedex, France), which allowed the anatomic parameters to be measured for each different specimen and neck cut level.

Measured from this reference point, the resection was performed at a distance of +10 , +5, and 0 mm (+5 mm is recommended by the manufacturer). The targeted angle of the resection was 50° in relation to the anatomic femoral axis. The bone was rasped until reaching the maximal rasp size as estimated by the senior surgeon.

A reference transmitter was attached to the greater trochanter by the means of a C-clamp. Afterward, a series of data was recorded using the navigation device. The transmitter attached to the greater trochanter was used as the frame of reference for all data acquisition. The first set of points was registered using a calibrated pointer at the bottom of the grooves in the fiducial screws. Next, when the senior surgeon determined that the appropriate rasp size was obtained and the final rasp size was in place, a calibrated adapter with a passive transmitter was attached to the rasp and the position of the rasp was recorded. After each final rasp size was recorded for each femoral neck resection level, 6 points of data were acquired from the perimeter of the femoral neck resection surface to define the resection plane. This procedure was repeated for each femoral neck resection (+10-, +5-, and 0-mm neck cut levels).

The acquired navigation data were then entered into CATIA (Version 5; Dassault Systemes, Suresnes Cedex, France) for further analyses. The segmentation data defined the outer contour of the femur as well as the femoral canal (Figure 2). The following anatomic parameters were measured: femoral neck anteversion, femoral offset, and the caput collum diaphysis (CCD) angle. Using fiducial points, the OrthoPilot data and the CT data sets were fused within CATIA. The following parameters were determined on the fused data sets for every level of the neck resection: rasp anteversion angle, CCD angle, anterior tilt of the rasp, and angle between the anatomic femoral axis and the resection plane. In addition, the best-fit adapter from the available portfolio (130°, 135°, and 140° of CCD angle with -7.5°, 0°, and +7.5° of anteversion, respectively) was defined, and the offset as well as leg lengthening using the best-fit adapter were determined.

Data analysis was then performed using spreadsheet statistical software. The results were tabulated to compare the changes in each variable with the specimen’s own native anatomy. A significance level of P=.05 was set using a paired t test to compare parameters at each neck cut level with the original anatomic parameters of each cadaver hip as determined from the preimplantation CT scan data.

Results

Anteversion

The +10-mm neck cut resulted in greater anteversion relative to the native anatomy (2°±5.1°). The +5-mm neck cut had higher anteversion (1.1°±3.6°). The 0-mm neck cut resulted in higher anteversion (0.2°±3.3°). All P values were >.22.

Femoral Offset

The +10-mm neck cut resulted in greater femoral offset relative to the native anatomy (4.7±3.4 mm) and was significantly different, with a P value of .001. The +5-mm neck cut also had higher femoral offset than the native anatomy (1±1.9 mm), but this was not statistically significant (P=.07). On average, the 0-mm neck cut had a lower offset compared with the native anatomy but again was not statistically significant (-1.7±3.1 mm; P=.072).

Caput Collum Diaphysis Angle

The CCD angle changes were the most variable from each neck cut investigated. The +10-mm neck cut resulted in a CCD angle that was significantly higher than the measured normal anatomy (5.6°±7.4°; P=.027). The +5-mm neck cut resulted in a significantly higher CCD angle (10.1°+6.9°; P=.001). The 0-mm neck cut also resulted in a significantly higher CCD angle compared with the native anatomy CCD angle (15.5°±7.2°; P=.0001).

Leg Length

When the leg length was calculated for a neutral neck length femoral head, the +10-mm neck cut resulted in an average increase in leg length (9±3.1 mm; P<.0001). A short neck choice would reduce this to a 6.2-mm average increase in leg length. For the recommended femoral neck cut level of +5 mm, the average leg length increased to 5.4±7.2 mm (P=.002) and would be decreased to an average of 1.6 mm with a short neck length. The 0-mm low neck cut resulted in an average increase in leg length of 2.9±3.7 mm with a neutral neck (P=.022); with a short neck, this would be decreased to an average of 0.1 mm.

Discussion

This study shows that the level of the neck resection with a short metaphyseal stem can significantly affect the final position of the stem as well as the CCD angle, femoral offset, and leg length. With the increasing number of orthopedic surgeons across the world turning to bone-preserving THA procedures, an effective technique to surgically implant these devices is paramount. This study shows that if the femoral resection level is at the shoulder of the femoral neck, the implant sits more vertically and cannot reproduce the femoral offset or the CCD angle. The higher the neck cut (+10 mm from the shoulder point), the longer the leg length, and in some cases the modularity options may not allow re-creation of the starting leg length for the patient. Significant technical issues need to be relayed to any surgeon thinking about using these types of implants.

A drawbacks of our study is that we did not take into consideration the changes in leg length and offset that would have resulted from the acetabular preparation for each of the specimens tested. This could have allowed a better re-creation of the leg length. We also did not investigate the large variation that could occur when different surgeons perform the procedures.

In a previous study by Confalonieri et al²² of 44 patients using the Metha implant and computer navigation, re-creation of leg length and offset was possible and was easier with computer assistance. Our study confirms those results in a more analytical fashion. However, the study by Confalonieri et al²² does not address whether the problems with offset, CCD, or leg length would be realized if computer navigation was not used.

Our results show that the best combination of variables that most closely re-created the specimen’s native anatomy included the recommended level of resection of 5 mm from the shoulder of the femoral neck. This resulted in an implant position that yielded 1 mm of increased offset, an average of 1.6 mm increased leg length with a short neck option, a CCD angle that could be compensated for with an optional neck angle to within 5°, and an anteversion angle that was re-created within 1.1° on average.

Most younger patients who undergo THA have unilateral disease and may feel the effects of unequal leg length or offset for many years.^4,5,7,11 This may influence how they interpret their surgical result. Knowing the trends on how to re-create the anatomic parameters is important. It can be difficult, however, in many of these cases because the starting anatomy may be abnormal. The surgeon, therefore, may have difficulty assessing how the implant is changing the resulting anatomy. It would be beneficial in these cases to use computer navigation to track how the parameters are changing during the case to give the patient the best possible outcome. In combination with intraoperative planning tools to assess the musculoskeletal loading conditions at the hip, navigation systems could provide intraoperative decision support and contribute to securing a high standard in patient treatment with short-stemmed modular implants.^23-25

Conclusion

The findings of our study suggest that the level of the femoral neck cut and the anatomic shape of the bone dictate implant positioning of a short metaphyseal stem. The lower the neck cut, the more vertical the implant positions itself, which decreases offset, increases the CCD angle, and drives the rasp closer to the anatomic anteversion. Care must be taken to properly resect the appropriate amount of head and neck when using these types of implants. Modular necks of the Metha stem (-7.5°, neutral, and +7.5° of anteversion) make it possible to adjust for anatomic bone variations, providing restoration of the offset and other biomechanical parameters. The true estimation of the position is difficult according to this study. This is especially true when considering the full range of available modular options.

The conclusions of this study are specific to preoperative planning and intraoperative technique. Using metaphyseal stems is a much different technique than what most surgeons have been trained for in the past. A primary standard type of implant for a THA requires that the surgeon ensures that the broach has a lateral starting point on the proximal femur, which removes the bone at the base of the greater trochanter and lateral aspect of the femoral neck (the shoulder region). The broach is started in the position to prevent varus placement of the implant. The metaphyseal type stem requires the preservation of the lateral bone at the base of the femoral neck (the shoulder region) and the avoidance of any valgus positioning of the broach during the preparation of the stem. If the femoral neck resection level is too low, the broach will naturally move into a valgus nature, making offset difficult to restore. If the femoral neck resection is too long, the leg length may be difficult to restore. Given these findings, it makes sense that initial use of this stem be coupled with the use of fluoroscopic imaging in the operating room or with imageless navigation to aid the surgeon in using the most suitable components.

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Authors

Dr Mihalko is from Campbell Clinic Orthopaedics, InMotion Orthopaedic Research Laboratory, University of Tennessee, Memphis, Tennessee. Dr Saleh is from the Department of Surgery, Division of Orthopaedics, University of Southern Illinois, Springfield, Illinois. Dr Heller and Mr König are from Julius Wolff Institut, Charité – Universitätsmedizin Berlin, Germany. Mr Mollard is from Aesculap Incorporated, Grenoble, France. Dr Kammerzell is from Aesculap Inc, Bethlehem, Pennsylvania.

Drs Mihalko, Saleh, and Heller are consultants for Aesculap. Drs Saleh and Heller received research grants from Aesculap. Dr Heller is on Aesculap’s speakers’ bureau. Dr Kammerzell is an employee of Aesculap Implant Systems. Mr Mollard is an employee of B. Braun Aesculap. Mr König has no relevant financial relationships to disclose.

Correspondence should be addressed to: William M. Mihalko, MD, PhD, Campbell Clinic Orthopaedics, 1458 West Poplar Ave, Ste 100, Collierville, Tennessee 38017.

doi: 10.3928/01477447-20090915-53