Component malposition in hip resurfacing can cause early failure through the 2 main failure modes: femoral neck fracture and wear-related failures. The femoral component should be aligned in slight valgus with no notching of the superior femoral neck. Varus placement and notching are strongly correlated with early femoral neck fracture. We have developed special mushroom templates to assist with accurate femoral component sizing and positioning. Issues related to radiographic magnification, ruler measurements, and plastic overlay templates are avoided. Acetabular component malposition can result in wear-related failures. The geometry of most resurfacing sockets is such that the wall is thinner at the rim than at the apex. This is done to improve range of motion before impingement and results in the bearing having less coverage and a higher inclination than one would expect from the radiograph. Surgeons should therefore aim for a lower inclination than they would with total hip replacement. Acetabular anteversion, if excessive, can reduce the available contact area, and lead to edge wear and subluxation. Dysplasia, which is more common in women, can also result in excessive femoral anteversion. Taken together, the combined acetabular and femoral anteversion should not exceed 45°. Pseudotumors have been reported adjacent to metal-on-metal bearings. These reports point out that pseudotumors are often associated with component malposition. It is thought that edge wear due to malposition results in excessive metal debris, which is locally toxic. Proper component position is necessary for the short- and long-term success of hip resurfacing.
Component malposition is important in hip resurfacing because it affects 2 main failure modes: femoral neck fracture, which occurs early, and late failures due to component wear.
The femoral component should be placed in slight valgus relative to the native neck-shaft angle. This has the effect of reducing tensile stresses on the superior neck and decreasing the risk of femoral neck fracture.1 Excessive valgus, however, may lead to notching of the superior neck, which is a strong predictor of femoral neck fracture. To avoid notching, the surgeon may choose a larger femoral component that may result in excessive acetabular reaming. The ideal amount of valgus is approximately >5° to 10° more of the patients anatomy, although neutral position may be acceptable, particularly in femoral necks that are already in valgus. Varus placement of the femoral component, however, must be avoided, as this is associated with femoral neck fracture (Figure 1). It is also necessary to avoid inferior translation, which risks femoral neck notching and thus, fracture.
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Figure 1: The femoral component in varus (A), and with notching of the femoral neck (B). A femoral neck fracture is the result.
There are inherent difficulties involved in accurate placement of the femoral component. Reliance on measurements based on radiographs and plastic overlay templates will lead to malposition if there are differences in the actual and expected radiograph magnification. Traditional jigs utilize a stylus to assess neck diameter, yet there is wide variation in head/neck ratio. Several different femoral component sizes may be appropriate for a given neck diameter, yet only one will best match the head diameter. We have developed a system of mushroom templates (Figure 2) that allow the best head size to be chosen, and also allow placement of a target pin for use with the McMinn short-arm jig (Birmingham Hip Resurfacing System, Smith & Nephew, Memphis, Tennessee). Other than using the neck sizing guides, no other templates are necessary.
Figure 2: Mushroom templates for femoral component sizing and positioning. The mushroom template is used to visually assess head size and in conjunction with the neck sizer to choose the best-fitting femoral component. The chosen mushroom is then placed over the femoral head and tilted into slight valgus. A pin is drilled through the slot for use with the short-arm jig (Birmingham Hip Resurfacing System, Smith and Nephew, Memphis Tennessee). This allows for accurate femoral component sizing as well as positioning.
Femoral anteversion can be problematic in hip resurfacing. A common finding, especially in men, is retroversion of the arthritic femoral head on an appropriately anteverted neck. Anterior neck osteophytes are often present, with femoroacetabular impingement. This must be corrected to restore normal biomechanics. In resurfacing this type of femoral head, almost no bone will be removed from the head anteriorly, and more will be removed posteriorly as the new head position is aligned with the femoral neck. Failure to correct this head retroversion will result in loss of flexion, impingement, and potentially subluxation and edge wear.
Hip dysplasia is commonly associated with increased femoral anteversion. In this case, the head is centered on the neck, but the neck itself is too anteverted. Hip resurfacing, unlike total hip replacement (THR), does not afford the opportunity to correct neck anteversion. Combined with excessive acetabular anteversion, this can result in decreased contact area between the head and the socket, increased wear, edge wear, and subluxation. For this reason, the combined anteversion of the femoral neck and acetabulum should not exceed 45°.2 If it appears that it will exceed, THR is preferred. The preponderance of dysplasia in women, as well as their smaller size with less tolerance for malposition, may explain the higher failure rate of hip resurfacing in women. In cases of dysplasia, it is important to take excessive femoral anteversion into account when positioning the socket.
Correct positioning of the acetabular component is more important than the femoral component for long-term success of the implant. The socket geometry in most hip resurfacing devices available today is complex. The acetabular wall is thinner at the rim, and thicker at the apex. This is done to increase range of motion before impingement, by lateralizing the center of rotation of the hip by a few millimeters. However, it also decreases contact area, so positioning must be accurate. This also makes the inclination of the bearing more vertical than the apparent inclination seen on radiographs. This is unlike THR, and is not well taught to surgeons learning hip resurfacing.
Figure 3 shows a schematic of a typical hip resurfacing. Curve A is the outer diameter of the socket, which is 180°, and is what one sees on a radiograph. Curve A has a center of rotation at point A, where one would expect it to be in THR. However, the actual center of rotation of a hip resurfacing is different. The socket has an inner curve B, which is much less than 180°, perhaps 163°, due to the geometry of the socket. Its center of rotation is at point B, which coincides with the center of rotation of the femoral head component, a few millimeters more lateral than point A. This places the inclination of the bearing several degrees more vertical than one would expect on the radiograph. In Figure 1, the radiograph appearance shows a 45° inclination, but the bearing is actually at 53°. Metal-on-metal bearings cannot tolerate vertical socket placement, with associated edge wear, and high metal ion concentrations.3 The surgeon must therefore aim for a lower acetabular inclination than may be customary with THR. However, insufficient anteversion can cause psoas tendonitis.
Figure 3: The influence of hip resurfacing socket geometry on bearing inclination. Curve A, a 180° arc seen on radiography, is the outer diameter of the socket, with an apparent center at point A. The socket is thinner at the edges, and thicker at the apex, so the bearing, shown as curve B, has a more lateralized center of rotation at point B. This is also the center of rotation of the head. Thus, what may appear to be a 45° inclination on radiography is actually much more vertical, with the bearing inclination at 53° in this example.
Reports of pseudotumors in association with metal-on-metal bearings have consistently stressed the importance of component malposition. Pandit et al4 reported 20 pseudotumors in 17 patients, all women, and many whose implants were apparently malpositioned. Hart et al5 reported on patients with painful hip resurfacing, and found that more than half had periprosthetic lesions, and of those almost all were malpositioned. Recently, a retrieval study of bearings associated with pseudotumors has shown edge wear in every case,6 as well as increased overall wear. It is thought that edge wear due to malposition results in excessive metal debris, which is locally toxic.
It seems clear that component malposition can predispose a hip resurfacing patient to failure. Malposition on the femoral side can result in femoral neck fracture, and on the acetabular side can cause accelerated wear and possibly pseudotumors. Improvements in instrumentation, specialized templates, good teaching, and a better understanding of the implant geometry can help surgeons avoid these complications.
- Anglin C, Masri BA, Tonetti J, Hodgson A, Greidanus NV. Hip resurfacing femoral neck fracture influenced by valgus placement. Clin Orthop Relat Res. 2007; (465):71-79.
- McMinn D, ed. Modern Hip Resurfacing. London, UK: Springer; 2009.
- De Haan R, Pattyn C, Gill HS, Murray DW, Campbell PA, De Smet K. Correlation between inclination of the acetabular component and metal ion levels in metal-on-metal hip resurfacing replacement. J Bone Joint Surg Br. 2008; 90(10):1291-1297.
- Pandit H, Glyn-Jones S, McLardy-Smith P, et al. Pseudotumours associated with metal-on-metal hip resurfacings. J Bone Joint Surg Br. 2008; 90(7):847-51.
- Hart AJ, Sabah S, Henckel J, et al. The painful metal-on-metal hip resurfacing. J Bone Joint Surg Br. 2009; 91(6):738-44.
- Kwon YM, Glyn-Jones S, Simpson DJ, et al. Analysis of wear of retrieved metal-on-metal hip resurfacing implants revised due to pseudotumours. J Bone Joint Surg Br. 2010; 92(3):356-61.
Dr Brooks is from Cleveland Clinic, Cleveland, Ohio.
Dr Brooks is a consultant for Smith & Nephew and Stryker.
Presented at Current Concepts in Joint Replacement 2009 Winter Meeting; December 9-12, 2009; Orlando, Florida.
Correspondence should be addressed to: Peter Brooks, MD, FRCS(C), Cleveland Clinic, 9500 Euclid Ave A41, Cleveland, OH 44195 (email@example.com).