Hip Resurfacing

Total hip arthroplasty (THA) is among the most successful of elective surgical interventions; it has enabled drastic improvements in function and quality of life for millions of patients. THA is indicated for advanced hip disease resulting from osteoarthritis (OA), avascular necrosis (AVN), slipped capital femoral epiphysis (SFCE), congenital hip dysplasia, and rheumatoid arthritis (RA).

Conventional hip replacement has the potential risks of postoperative dislocation, bone loss extending to the metaphyseal and diaphyseal femur, and gross migration of the femoral component. Stress shielding of the proximal femur can occur with rigid intramedullary fixation of stiff metallic THA stems. Conventional joint replacement does not provide a lasting solution to suit the dynamic lifestyle of a young active population. Revision surgery with conventional joint replacement is inevitable in the young population and is associated with high morbidity.

Hip resurfacing is an alternative form of hip arthroplasty that conserves proximal femoral bone.[1, 2, 3, 4, 5, 6] Bone conservation is attractive to young and active patients. Other advantages of hip resurfacing over conventional THA include added hip stability from the larger diameter of the prosthetic femoral component and more accurate restoration of leg length, femoral offset, and femoral anteversion.

Thus, proponents of hip resurfacing claim that its advantages include a greater range of motion (ROM) of the hip joint after surgery, a lower rate of dislocation, less prosthetic bearing wear, easier revision surgery, and less likelihood of stress-shielding of the proximal femur. Hip resurfacing may therefore be particularly suitable for young and active patients.[7]

The short-term failure of hip resurfacing from the 1960s through the 1980s was due to aseptic loosening, which was caused by poor internal fixation, generation of large volumes of biologically active particulate debris (leading to bone loss), technical errors in device implantation, and problems with design and manufacturing.

Volumetric polyethylene wear in initial metal-on-polyethylene resurfacing was often 10 times higher than for standard hip replacements. Blood and urine metal ion levels, metallosis, capsular lymphocytic aggregation, and hypersensitivity are still major concerns with metal-on-metal articulation.

The major mode of failure of first-generation hip resurfacing procedures was femoral neck fracture, caused by improper surgical technique, or osteolysis of the femoral neck from wear particles produced by the articulation. Femoral bone stock was well preserved in first-generation hip resurfacing implants, but significant acetabular bone was removed to accommodate the acetabular component and the cement mantle.

Only comparatively recently has resurfacing arthroplasty regained acceptance, with successful short- and medium-term results being achieved through design modifications and improvements in surgical technique (long-term safety and efficacy and revision surgery outcomes require further evaluation).

Hip resurfacing is being performed more frequently in the United Kingdom and Australia than in the United States. It has been reported to account for 7.5% of all hip replacements in Australia and to have a 2.2% annual revision rate, with femoral fracture being the most common reason for revision. In a comparison study, however, UK researchers found no evidence that hip resurfacing provided better hip function or higher activity levels than total hip replacement in the first year after surgery.[8]  

A key attraction of hip resurfacing is that it addresses both the increasing expectations of the active older patients and the work and recreational activities of younger patients. It must be kept in mind, however, that the procedure should be performed only by surgeons trained in resurfacing. Ceramic bearings are currently precluded, because the thickness of ceramic required would not be bone-sparing on the acetabular side. In the future, ceramic-on-ceramic resurfacing may offer better wear properties and address the issues of metallosis and metal hypersensitivity.

Suitable candidates for resurfacing are patients younger than 60 years with good bone stock and a diagnosis of OA.[9] Resurfacing can be done in patients with proximal femoral deformity when a standard stemmed prosthesis is difficult to place. It is a good alternative for patients with neuromuscular disorders because it is associated with less risk of hip dislocation.

Resurfacing is also an option in cases involving retained hardware that would be difficult to remove for a standard stem placement. It can be a choice in patients with conditions that pose a high risk for failure after standard THA, such as sickle-cell disease and alcoholism.

Advanced hip arthritis usually manifests with radiographic evidence of joint-space narrowing in the hip, osteophytes, periarticular cysts, and subchondral sclerosis. Typically, the spherical shape of the femoral head and the acetabular cavity is preserved or deformed slightly as a consequence of mechanical wear and secondary skeletal remodeling. Ideally, for hip resurfacing to be considered, there should be no significant anatomic deformity of the femoral head and acetabulum, particularly the former.

If the femoral head can be reamed so that the spherical contour can be reconstructed and if there is sufficient viable subchondral bone, then hip resurfacing is a reasonable option in the appropriate patient population. Mild dysplasia of the acetabulum can be reamed away, but a deformed femoral head may require excessive reaming and bone removal to seat a resurfacing femoral component.

Cystic degeneration of the femoral head and collapse after severe long-standing AVN can lead to anatomic deformities that can preclude hip resurfacing. In such cases, THA components should be available during surgery as a backup, and the patient should know that a THA may be performed instead.

Absolute contraindications for hip resurfacing include advanced age and postmenopausal status with osteoporosis, impaired renal function, and known metal hypersensitivity. Other contraindications include deficiency of the femoral head or neck bone stock, femoral neck or head cysts, severe hip dysplasia, and a small or bone-deficient acetabulum.

A hip with deficient femoral head bone stock after fracture or other conditions (eg, rapidly progressive OA, or disappearing bone disease) cannot be resurfaced. Hip resurfacing cannot correct large inequalities in limb length or change horizontal femoral offset; accordingly, arthritic hips that are at least 1 cm shorter than the contralateral limb or that have low horizontal femoral offset may be better managed with a standard THA or extended offset stemmed implants.

Relative contraindications include inflammatory arthropathy, abnormal proximal femoral geometry, and extensive osteonecrosis. For successful implantation of the femoral component, the spherical shape of the femoral head must be preserved. Deformation of the femoral head resulting from severe trauma, hip dysplasia, extensive cystic degeneration, previous surgery or hardware, extensive collapse from AVN, and related conditions precludes hip resurfacing, making THA a more reasonable option. Selected patients with femoral head osteonecrosis may be treatable with hip resurfacing; care is warranted with large necrotic lesions.[10]

Procedural planning

Advantages of hip resurfacing

Hip resurfacing has the following advantages over conventional THA:

• It restores physiologic biomechanics
• It preserves bone stock
• It allows the patient to return to a more active lifestyle - Favorable survivorship with a return to high-demand activities after hip resurfacing may be advantageous in younger patients; however, patients should limit their involvement in sports to realistic levels that the implant will be able to tolerate ^[11]

Although outcomes data on functional results from hip resurfacing vs replacement have been mixed, proponent surgeons believe that patients who undergo hip resurfacing have better kinematics and higher activity scores than those who undergo standard THA. They walk faster and have better hip abduction and postoperative ROM in gait analysis. The physiologic pattern of femoral head loading after resurfacing facilitates better stress transfer by producing compressive forces rather than hoop stress. This may improve bone mineral density (BMD), prevent stress shielding, preserve the proximal femoral bone stock, and avoid periprosthetic bone loss.[12]

The use of metal-on-metal bearings may lead to marked reductions in periprosthetic bone loss, which was often extensive with first-generation polyethylene acetabular resurfacing components. Advances in design have led to the introduction of polar bearing implants, which have reduced surface asperity and provided superior fluid film lubrication for the bearing surfaces. The thin acetabular shells that have been developed allow the removal of less acetabular bone.

Hip resurfacing, by accommodating a large femoral head, is associated with a lower dislocation rate than conventional THA is. Finally, hip resurfacing may yield better proprioception than conventional THA because of preservation of proximal femoral bone.

Modern hip resurfacing devices commonly consist of a cemented femoral component and a hydroxyapatite-coated acetabular component. Contemporary components have a metal acetabular bearing that requires the removal of little acetabular bone stock. Resurfacing preserves bone on the femoral side and avoids use of the intramedullary devices that are implanted in standard hip replacements. Resurfacing with a highly cross-linked polyethylene acetabular component and a titanium nitride-coated titanium cementless femoral component has been described.[13]

With femoral failure, the acetabular component can be left in place and mated to a standard femoral stem with a large-diameter head. Revision of hip resurfacing is thus easier than revision of a failed THA. Other intraoperative advantages of hip resurfacing may include less blood loss and a reduced need for blood transfusion because the intramedullary femoral canal is not broached. The BMD of the proximal femur is maintained or increased with resurfacing, whereas a significant decrease from stress-shielding can occur with THA.

Hemiresurfacing of the hip refers to resurfacing the femoral head alone while preserving the native acetabulum. It yields unpredictable pain relief, with 80% and 60% survivorship at 5 and 10 years, respectively. Acetabular cartilage wear or persistent pain is the primary cause for failure. Although these outcomes are inferior to those of standard THA performed by a skilled surgeon, they are comparable, and perhaps superior, to those of other femoral head-sparing procedures (eg, free vascularized fibular graft and proximal femoral osteotomy) and result in less morbidity.

Limitations of hip resurfacing

For resurfacing to be truly conservative, the surgeon should not remove any more bone from the acetabulum than would be required for a THA. Only relatively recently has this goal been addressed by the development of thinner acetabular shells that are designed for implantation without cement. For patients with low femoral head-neck ratio (< 1.2), such as those with OA secondary to Legg-Calvé-Perthes disease or SCFE, removal of more acetabular bone is often necessary during hip resurfacing.

The lack of modularity of resurfacing devices reduces the ability to adjust leg length. The biomechanical results of hip resurfacing have depended on the preoperative anatomy of the femur. Limb lengthening cannot be achieved, and horizontal offset is essentially unchanged by resurfacing. In contrast, both offset and limb lengthening can be increased reliably during THA. A biomechanical study recommended that leg-length discrepancy be no greater than 1 cm and that the hip should have high horizontal femoral offset.

For hip resurfacing, body mass index (BMI) should be less than 35 kg/m2, though some studies have shown no prejudicial effect of BMI on the outcome after hip resurfacing. No difference in rehabilitation has been shown between hip resurfacing and THA.

Some patients may experience a noise after resurfacing; one study showed a 23% prevalence of clicking and a 4% prevalence of squeaking after metal-on-metal resurfacing. It is unclear if these joint noises are related to impingement, subluxation, or microseparation of the components.

Metal-on-metal resurfacing should be used with caution in women who want to become pregnant after hip arthroplasty and patients with definite metal allergy or mild compromise of renal function, even though a study of three women with metal-on-metal replacements and elevated ion levels showed no increase in ion levels in the umbilical cord blood and no adverse consequences for the infants.

Nevertheless, the elevation of serum metal ions remains a potential long-term concern for all patients,[14] and until further studies clarify the significance of this phenomenon, any metal-on-metal bearing in the body should be implanted cautiously and only after proper patient counseling. In addition, pseudotumors have been shown to occur in the periprosthetic joint space with metal-on-metal hip resurfacing,[15] raising the concern that adverse host reactions to metal ion debris may be more widespread and a greater concern than earlier studies have identified.

Finally, no study has shown hip resurfacing to be superior to modern total hip replacements with regard to durability and survivorship.

Some data suggest that modern hip resurfacing has a survival rate of 98% at 5-year follow-up. The most common reason for revision was fracture of the femoral neck, mostly occurring within the first 6 months. Men older than 65 years are at higher risk for revision than men younger than 65 years. Among women, those aged 55-64 years are at highest risk for revision. Patients with OA have a lower incidence of early revisions than patients with RA, osteonecrosis, or developmental dysplasia.

An article on short-term survivorship of resurfacing arthroplasty suggested that the results are equivalent to those of traditional THA. It should be emphasized that the study was from the Finnish Registry and was limited to 3-year follow-up. A short-term study, based on registry data, could only suggest early patterns of behavior rather than make definitive statements about expected outcomes.[16]

In a retrospective cohort study aimed at comparing 10-year cumulative mortality figures for patients undergoing metal-on-metal hip resurfacing and those undergoing cemented or uncemented THA for OA in England from April 1999 to March 2012, Kendal et al determined that metal-on-metal hip resurfacing was associated with reduced long-term mortality as compared with either variety of THA.[17] The difference remained after confounding factors were adjusted for.

ROM is improved in all patients after hip resurfacing, with reported mean flexion of 92° preoperatively and 110° postoperatively.

At 5-year radiostereometric follow-up, resurfaced hips have shown no changes in mean acetabular component translation and rotation and femoral component translation, indicating that the implants are stable (at least over a 5-year period).

Some narrowing of the femoral neck can occur after resurfacing, and this phenomenon is not associated with any adverse clinical or radiologic outcome up to a maximum of 6 years after the initial operation. There is a significant association between narrowing in females and a valgus femoral neck-shaft angle. There is no significant association between the range of movement, position or size of the component or radiologic lucent lines, and narrowing of the neck.

Daniel et al reported a 99.8% survival rate with a follow-up of 1-8 years for 446 hip resurfacings performed in 384 OA patients using the Birmingham hip resurfacing (BHR) approach.[18]

Treacy et al reported a series of 144 consecutive metal-on-metal resurfacings of hips (BHR) with a survival rate of 98% at a minimum of 5 years' follow-up.[19]

Amstutz et al reported on a series of 400 hip resurfacings performed in 355 patients using the Conserve Plus prosthesis and documented a 94% component survival rate at 4 years.[20] They also presented the risk index for failure.

Gravius et al described a series of 59 resurfacings with the Durom prosthesis in 52 patients and reported two revisions over a period of 25 ± 10 months, which were attributable to femoral neck fracture and heterotopic ossification (HO).[21]

Nishii et al evaluated the 5-year results of metal-on-metal resurfacing arthroplasty in 45 patients with developmental dysplasia or dislocation of the hip (70%) and noted a 96% 5-year survival rate.[22] They reported two revisions, attributable to femoral neck fracture and septic loosening; they also had one femoral component loosening.

Steffen et al, in a study evaluating 5-year clinical follow-up and 7-year survival in a total of 610 BHR procedures done in 532 patients, revised 23 hips (3.8%), mainly because of femoral neck fracture (12), aseptic loosening (four) and possible metal debris (three).[23] They reported a 93% rate of excellent or good outcome (on the Harris hip score) at 5 years and 95% survival at 7 years. There were no patients with definite radiologic evidence of loosening or of narrowing of the femoral neck exceeding 10% of its width.

Hing et al, in a study evaluating the results of 230 BHR procedures in 212 patients at a mean follow-up of 5 years, reported a survival rate of 97.8% and noted that two revisions were done because of acetabular component loosing and suspected avascular femoral head necrosis.[24]

Radiologic review at 5-year follow-up showed one patient with progressive lucent line around the acetabular component and six with progressive lucent lines around the femoral component.[24] About 8% of femoral components migrated into varus, and those with lucent lines migrated more than the rest. The investigators reported a statistically significant association between the presence of radiolucent lines to superolateral notching of the femoral neck and reactive sclerosis at the tip of the femoral peg.

Amstutz et al evaluated metal-on-metal hybrid hip resurfacing for Crowe type I and II developmental dysplasia in 51 middle-aged patients (59 hips), concluding that medium-term results were disappointing with respect to femoral component durability. However, the fixation of the porous-coated acetabular components without adjuvant fixation was excellent despite incomplete lateral acetabular coverage of the socket. More rigorous patient selection and meticulous bone preparation are essential to minimize femoral neck fractures and loosening after this procedure.

Ball et al presented early results of conversion of 21 failed femoral components in 20 hip resurfacing patients and found no significant differences between the conversion arthroplasty group and the conventional arthroplasty group with regard to operating time, blood loss, or complication rates.[25] At 2-year follow-up, there were no significant differences between the two groups with regard to the mean Harris hip score; the University of California at Los Angeles pain, walking, and activity score; or the SF-12 score.

On radiographic assessment, the quality of component fixation and the alignment of the reconstruction were equivalent between the two groups.[25] There had been no instances of aseptic loosening of the femoral or the acetabular component in either group, and there had been no dislocations after conversion of a resurfacing arthroplasty.

Pascual-Garrido et al retrospectively reviewed the outcomes of 202 patients (206 hips) treated with BHR procedures (mean patient age, 51 ± 8 years; mean follow-up, 4 ± 1.6 years).[26] Outcomes were evaluated by recording the Harris Hip Score at 6 months, at 12 months, and yearly thereafter. The BHR procedure yielded significant postoperative improvement was significant, from a score of 62.9 ± 10.6 before surgery to a score of 98.6 ± 6.7 postoperatively. Nine patients (4%) had complications, and five hips (2.4%) underwent revision surgery. Overall, men had better survival rates and outcomes than women (especially those with modest bone size)

Brooks reported a large (N = 1333; mean age, 53.1 years) single-surgeon experience with the BHR system over a mean follow-up period of 4.3 years (range, 2-5.7 years). Of the 1333 patients 70% were male and 91% had osteoarthritis. Complications rates were low, and no destructive pseudotumors occurred. At up to 5.7 years, overall survivorship was 99.2%, and in males younger than 50 years, aseptic survivorship was 100%.[27]

Costa et al, in a study of 122 young patients with arthritis of the hip joint, compared hip function and quality of life after THA (n = 62) and resurfacing arthroplasty (n = 60) in the medium term.[28]  Of the original 122 patients, 95 provided data at 5 years. There was a small decrease in both hip function and quality of life in both groups of patients each year during the 5-year follow-up period; however, no significant differences between the two groups were noted.

Maillot et al, in a retrospective case-control study of 52 patients who had unilateral primary OA successfully treated by means of replacement, found that hip resurfacing was more consistently able to generate a more physiologic gait under stress conditions than conventional THA was.[29]

A study from the Australian Orthopaedic Association National Joint Replacement Registry compared the Birmingham Hip Resurfacing (BHR) prosthesis (n = 4790; mean age, 52 ± 7.8 y; mean follow-up, 11.9 y) with the three best-performing conventional THA prostheses (n = 4790; mean age, 56 ± 7.1 y; mean follow-up, 9.3 y) in males younger than 65 years who underwent hip surgery for OA.[30]  After 2.5 years, revision rates were higher in BHR patients.

Su et al reported on the outcomes of 255 primary BHR procedures at a minimum follow-up of 10 years.[31]  The 10-year rate of survival free from all-cause component revision was 92.9% for all hips and 96.0% for male patients younger than 65 years old at the time of surgery. Reasons for revision included femoral loosening, femoral neck fracture, pseudotumor, osteolysis, and acetabular loosening, as well as a combination of pain, noise, or metal levels. Among unrevised hips, the median Harris hip score improved from the preoperative period to 1 year postoperatively and remained stable for 10 years.

A routine preoperative workup consisting of laboratory evaluation (including blood work, urinalysis, and any other studies deemed necessary), complete medical workup, radiography, and electrocardiography (ECG) is necessary. Additional studies may be warranted, depending on patient comorbidities. No special workup is indicated for hip resurfacing patients as compared with standard total hip arthroplasty (THA) patients.

Preoperative templating is mandatory in planning a successful hip resurfacing. It is essential to obtain an anteroposterior (AP) radiograph of the pelvis (see the image below), a frogleg lateral view of the proximal femur, and a true lateral view of the hip joint so as to properly evaluate osteophytes, the femoral neck-shaft angle, and femoral neck anteversion. Deformity of the femoral head and cyst size and orientation should be evaluated, as well as leg lengths, femoral offset, and the overall shape of the femoral head and acetabulum.

If there is any doubt about leg lengths, computed tomography (CT) can provide definitive evaluation. The opposite hip joint should be evaluated on the AP radiograph for comparison of leg lengths and existing offset relations. In selected patients, other studies (eg, vascular indices, spinal radiographs, magnetic resonance imaging (MRI) of the hip or spine, and evaluation of the ipsilateral knee joint) may be required.

If there is any doubt about existing hip disease, MRI or a diagnostic injection into the hip joint may be useful. If a fluoroscopically guided hip joint injection relieves pain, then the pathology is probably localized to the intra-articular joint.

Careful examination and the use of selected diagnostic modalities may be needed to rule out spinal stenosis, herniated lumbar disk, vascular claudication, incarcerated hernia, meralgia paresthetica, snapping psoas tendon, hip bursitis, transient osteoporosis, malignancy, stress fracture, and other conditions that can mimic the pain and other symptoms of hip degeneration.

With regard to histologic studies, hip resurfacing (unlike THA, in which the retrieved femoral head can be examined histologically) yields specimens that are mechanically deformed as a consequence of reaming. Thus, only shavings from the acetabulum and the femoral head are typically available for pathologic analysis.

A retrospective study by Gaillard-Campbell evaluated the cost-effectiveness and safety of outpatient hip resurfacing in 485 procedures performed by two experienced surgeons.[32] Clinical outcomes, patient satisfaction, and average insurance reimbursement were compared with corresponding figures for hip replacement. No major complications occurred within 6 weeks. Of the 39 patients with previous inpatient experience, 37 (95%) considered their outpatient experience superior. The average payment for outpatient resurfacing was $26,000, compared with an average reimbursement of $50,000 for hip arthroplasty at local hospitals.

Patients typically require protected weightbearing with crutches for 6 weeks after surgery to allow soft tissues to heal and minimize the risk of a femoral neck fracture. Hip precautions are probably safest during this time at least, in light of the extensive dissection and disruption of the short external rotators inherent in the standard posterior approach to the hip joint. Long-term hip precautions are not necessary, because the large femoral head diameter protects against the risk of dislocation.

Before hip resurfacing is decided on, conservative treatment should be tried to control pain. Such treatment can include pain medications, reasonable exercises, weight loss, aerobic conditioning, and, if indicated, injection of steroid into the hip joint and arthroscopic debridement of the arthritic joint. If such treatments fail, if radiographic templating shows that resurfacing is a reasonable option, and if the patient is a suitable candidate, then hip resurfacing may be indicated.

The surgical approach used for hip resurfacing is typically a more extensive version of what is needed for a standard total hip arthroplasty (THA), in that the femoral head is preserved and must be sufficiently displaced to allow visualization of the acetabulum. The following are key considerations in planning and carrying out the surgical procedure.

Anterior vs posterior approach

Variants of the anterior approach have been advocated because they preserve the blood supply to the femoral head. However, most surgeons have used the posterior approach, both because they are familiar with it and because disruption of the blood supply to the femoral head with this approach does not appear to have an adverse effect on clinical outcomes from hip resurfacing.

A study by Tjur et al compared an anterolateral approach to hip resurfacing arthroplasty with a posterior approach and found that at 1 year after the procedure, the former approach was associated with reduced frontal plane moment during early stance as compared with the latter approach.[33]

Sizing

Sizing considerations are different in hip resurfacing. In the course of reaming the acetabulum, sufficient bone must be taken to allow placement of an appropriately sized femoral component without notching of the femoral neck. Placement of too small an acetabular component necessitates a smaller femoral component, which may lead to unwanted notching of the femoral neck.

Fortunately, most modern acetabular designs are thin enough that excessive reaming of acetabular bone is not needed. Modern acetabular components are also designed for cementless fixation to the pelvis, using a press fit to achieve initial skeletal stability. Femoral components are always cemented in place. Cement-mantle thickness between the cylindrically reamed part of the femoral head and the component varies in different designs, and a lower-viscosity acrylic cement is recommended for femoral components with lower clearance.

Some components are designed to allow fine adjustments for individual anatomy. Thus, acetabular components are available for supplementary screw fixation in congenitally dysplastic acetabula. In severe hip osteoarthritis (OA) secondary to acetabular fractures, a thicker metal cup can allow restoration of lost limb length as well.

One-stage vs two-stage procedure

A study comparing one-stage bilateral total hip resurfacing with two-stage resurfacing (in which a single hip resurfacing was done first and the second delayed until later) with respect to perioperative complications, transfusion requirements, hospital stay, outcome, and costs found that the complication rate was 8.1% in the one-stage group and 1.8% in the two-stage group.[34] There were no significant differences in the transfusion requirements.

The one-stage group had a reduced total hospital stay (5 days) and a reduced length of time to completion of all surgery (5 months).[34] The cost was 35% less than that of a two-stage procedure. However, the total anesthetic time was significantly longer for the one-stage group. Consideration should be given to one-stage surgery for patients with bilateral symptomatic disease suitable for metal-on-metal hip resurfacing, provided that the patient is a suitable candidate for extensive surgery.

A single-center, single-surgeon retrospective cohort study evaluated costs in patients who underwent bilateral hip resurfacing either in one stage (n = 53; 106 hips) or in two (n = 77; 154 hips), followed for a mean or 4.8 years.[35]  The main differences between groups were in cost and duration of hospitalization: in the one-stage group, total cost was €10,546.06 ± 2049.47 and average length of stay 9.83 ± 2.19 days, whereas in the two-stage group, total cost was €13,569.49 ± 2186.30 and average length of stay 15.04 ± 3.15 days. There was no significant difference bettwen the two groups with respect to the length of medical leave.

Operative details

During the operation, the extremity undergoing resurfacing should be draped free so that the surgeon can move it enough to dislocate the femoral head and then turn it sufficiently to allow expansive exposure of the acetabulum. This will also require extensive dissection of the hip capsule; in hip OA, circumferential release of the capsule will usually be necessary. Patient positioning should be planned to allow this extent of surgery.

Some resurfacing designs also allow venting of the intramedullary proximal femur, thereby requiring exposure of the lesser trochanter. In all hip resurfacing operations, a back system to convert to THA should be available in case intraoperative findings argue against a resurfacing procedure. In very stiff hips that undergo resurfacing through the posterior approach, consideration should be given to releasing the gluteus maximus insertion into the femur; doing this can relieve tension on the sciatic nerve and may be warranted in stiff hips.

Femoral neck fracture

The primary modes of failure in resurfacing are femoral neck fracture and femoral implant loosening. Femoral neck fracture is a unique complication of resurfacing. Its reported incidence after hip resurfacing is approximately 1.3% (range, 0-10%). The mean time for femoral neck fracture is 15 weeks after surgery. In women, the fractures are more likely to have been preceded by a prodromal phase of pain and limping. The relative risk of fracture is twice as high in women as in men.

Patient-related factors for femoral neck fractures include obesity, decreased bone mass, inflammatory arthritis, and femoral neck cysts. A body weight lower than 65 kg was significantly associated with a smaller femoral component size and a smaller fixation area and may result in adverse radiographic changes.

Obesity contributes to poor surgical exposure and osteoporosis, which may contribute to an increased risk of femoral neck fracture. Postmenopausal women may have osteoporosis. Other risk factors include overaggressive removal of the large anterior femoral neck osteophyte, reaming, osteopenia, cystic degeneration, aggressive impaction of the femoral component, and overpressurized insertion of acrylic into poor-quality bone. Avascularity caused by hip exposure or dislocation and femoral component cementing techniques may be a related factor.

Fractures are rare when the femoral component is placed in valgus compared with the preoperative neck-shaft angle. Valgus placement of femoral components optimizes the loadbearing capacity of the femoral neck. Varus placement increases the tensile stress on the superior cortex and the medial compressive forces, allowing shear to develop at the prosthesis neck junction.

A study showed complete recovery of bone mineral density (BMD) of the femoral neck 2 years after surgery in patients with valgus alignment of the femoral component. Conversely, BMD was lower in patients with varus alignment. Bone resorption can be minimized by a valgus position of the femoral component, which may be the ideal alignment for preventing femoral neck fracture in the early postoperative period.

Technical problems encountered during surgery were found retrospectively in 85% of hip resurfacings with subsequent femoral neck fractures. Surgical errors, such as impingement, notching of the prosthesis, tilting of the prosthesis into excess varus (< 130º neck-shaft angle), and improper prosthetic seating or malalignment of the implant, increase the risk of fracture.

The incidence of femoral neck fractures declines as the surgeon gains experience. Most femoral neck fractures are preventable by selecting patients carefully and employing optimal surgical technique. Femoral notching, possible stress fracture, and nondisplaced femoral neck fractures that are detected early can be successfully treated with a period of protected weightbearing or avoidance of weightbearing.

Femoral implant loosening

The important risk factors for femoral component loosening include large femoral head cysts, patient height, female sex, and smaller component size in male patients. A major reason for failure was apparently inadequate fixation at the time of surgery secondary to poor bone preparation or a small area of fixation because of bone loss related to cystic defects.

Radiolucency was reported around the stems that were not cemented, but no radiolucencies were observed around the cemented stems. Aseptic loosing of femoral stems was attributed to intraoperative errors such as varus stem shaft angle, and large areas of cystic degeneration.

A 0.5% incidence of acetabular component loosening has been reported, typically from a lack of initial stability obtained during cup insertion and a failure to assess stability properly at the time.

Osteonecrosis

Hip resurfacing has been shown to affect the oxygen concentration in the femoral head when done via an extended posterior approach, causing an average of 60% decrease in oxygen concentration. Component insertion results in an additional 20% decrease. Oxygen concentration does not improve significantly with wound closure, which raises concerns about the viability of the femoral head and neck after resurfacing.

It can be postulated that osteonecrosis may be a cause of some femoral neck fracture if the intraoperative reduction in oxygenation is permanent. Although osteonecrosis of the femoral head was proposed as a cause of failure, the viability of the femoral head may not necessarily be important for excellent results. Only 12% of failed specimens had osteonecrosis, and excellent results were reported in patients with osteonecrosis of the femoral head.

It remains to be determined whether femoral head viability has an impact on resurfacing. Histologic evidence of osteonecrosis has been a common finding in resurfaced hips that fail by fracture; however, this is changing. The posterior approach sacrifices the ascending branch of the medial femoral circumflex artery, which is an important contributor to the subsynovial supply of the femoral head. Nevertheless, the incidence of osteonecrosis is negligible. A possible reason is that it is actually the neck rather than the head that is being resurfaced.[36, 37]

Increased metal ions

No deterioration in renal function has been reported with modern metal-on-metal resurfacing in healthy individuals. Nevertheless, metal ion release is a valid concern with any metal-on-metal bearing in the human body, regardless of whether the bearing is in a hip replacement or a hip resurfacing device.

Although no long-term risk of cancer has been identified, the general recommendation is to make the patient aware of the potential unknowns regarding metal ion release and accumulation in the body. Routine monitoring of metal ions is not recommended, though this may become a valid concern in the patient who subsequently develops renal failure and cannot clear metal released from the bearing.

Metal hypersensitivity

Dermal hypersensitivity to metal affects 10-15% of the population, with nickel being the most common sensitizer and cobalt chromium the second most common. Metal hypersensitivity is more common in patients who have undergone arthroplasty than in the general population. It has been implicated as a cause of groin pain in some patients who have undergone hip resurfacing, but its relation to osteolysis and implant failure is unknown.

The role of metal ions in relation to delayed type and humoral sensitivity reactions in patients with metal-on-metal implants remains unknown. The histologic appearance of tissue samples from these patients is characterized by aseptic lymphocytic vasculitis associated lesions.

Heterotopic ossification

The risk of higher-grade heterotopic ossification (HO) may be greater after resurfacing arthroplasty than after conventional hip arthroplasty. HO was noted in approximately 60% of patients who had resurfacing without prophylaxis. It is more common in males.

HO is likely to be multifactorial, with muscle trauma and bone debris implicated as contributing factors. Other causative factors include membership in a higher-risk patient group (young males); failure to use prophylactic indomethacin routinely in high-risk groups; and the surgical approach used in this procedure, which is characterized by extensile exposure and has a learning curve that could have resulted in more soft-tissue stripping. Other risk factors include bilateral simultaneous hip resurfacing and previous heterotopic bone formation.

Although its effect on clinical outcome scores is minimal, HO may reduce range of motion (ROM). Indomethacin reduces the risk of HO formation, and the recommendations for its use vary from selective use in high-risk patients to routine prophylaxis in all patients. Prophylaxis with indomethacin after resurfacing is recommended.

Femoral neck narrowing

Anteroposterior (AP) and lateral radiographic evidence of femoral neck narrowing (loss of >10% of femoral neck diameter from the original postoperative radiographs) has been reported. Such narrowing is difficult to quantify in the AP plane because of the variable cross-table lateral radiographs. The only reliable information available regarding femoral neck narrowing, therefore, is obtained in the mediolateral radiographs.

Femoral neck narrowing varies with the type of prosthesis. Although its exact etiology and significance are unknown, possible causes include stress shielding caused by design and surface coating (reduction of femoral head diameter in hips that have been resurfaced with thick shells or hydroxyapatite coating) and external pressure resulting from synovitis. Progressive femoral neck resorption, though rare, can lead to femoral component loosening and failure, thereby necessitating repeat surgery.

Other complications

Complications associated with hip arthroplasty that also apply to resurfacing include dislocation, thromboembolic disease, nerve palsies, and vascular damage. A 2% prevalence of nerve palsy after 230 resurfacing procedures has been reported, which may be related to retraction and difficulties with exposure. Sciatic and peroneal nerve complications have been more commonly identified with the posterior-lateral approach. It is important to avoid pinching the nerve between trochanter and the ischium when the dislocated hip is positioned into extreme rotation.