Hip Resurfacing Technique

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.