Incidence of Femoral Neck Fractures

To assess the worldwide incidence of femoral neck fracture after modern hip resurfacing, we collected data from the reports providing this information since 2005 and selected the most recent when several reports were published based on the same series of patients. Our computations yielded a 1.69% incidence of femoral neck fracture (range, 0%–9.2%) for a global cohort of 10,381 cases, including 3497 from the Australian Hip Registry ( Table 1 ).

Etiology of Femoral Neck Fractures

The variability reflects the influence of surgeon experience, criteria for patient selection, component design, and surgical technique. Campbell and colleagues have performed extensive studies of failed MMSA. Failures ranged from 1 week to several years postoperatively. Of the 98 failures submitted for analysis, the most common cause of failure was femoral neck fracture (32%). Most of these fractures occurred within 2 months of surgery, but in 7 cases, the average time to fracture was 15 months. The procedure of resurfacing itself is thought to increase strains in the lateral aspect of the femoral neck and the lesser trochanter region. Shimmin and Back reviewed the resurfacing experience within the Australian National Registry of joint replacements, which includes predominately Birmingham hip resurfacing (BHR) (Smith and Nephew, Inc, Memphis, TN, USA) types, and reported that fractures of the femoral neck were twice as likely in women than men and were more prevalent when the femoral implant was placed in the varus. The effect of femoral component positioning on the potential risk of neck fracture was further studied by the finite element analysis or cadaver studies, and the consensus is that valgus placement is recommended to avoid femoral neck fracture, although Vail and colleagues suggest that small deviations from the anatomic placement of the component result in high localized stresses on the femoral neck.

The multifactorial nature of this complication was first highlighted by Amstutz and colleagues, who showed that most fractures that occurred in hips had more than 1 possible cause. Fractures that occurred early, within 2 months, were located at the neck component junction (subcapital), and the fracture of the hip, which occurred later (at 20 months post operatively), was located within the femoral head (intracapital) at the junction of viable and nonviable or repairing bone.

Notching has been reported in both fractured and successful resurfacings. Occasionally, stress fractures occurring under MMRA components have been reported to heal if noted early and if the hip is protected by a period of non–weight-bearing. Bone undergoing repair following stress fracture is a site of weakness for eventual fatigue failure.

Several incomplete (premonitory) and complete subcapital fractures were found unexpectedly by Campbell and colleagues after retrieval in resurfacing revised for pain or femoral migration ( Fig. 1 ). This case also illustrates the danger of over-pressurization of cement into the dome, which has been observed more often in those devices in which there is minimal or no cement mantle. Many of these failures can be considered to have a dual-phase failure mode; the original trauma to the bone occurs at surgery, but the actual failure process takes place several weeks or months later. Histologic analysis of the short-term fractures showed that the break occurred through areas of healing cancellous and cortical bone at the component/neck region. Similar findings were reported by Morlock and colleagues after the analysis of fracture patterns and histologic findings among 141 femoral resurfacing failures. Early fractures were associated with uncovered reamed bone and microfractures, with associated healing. The investigators speculated that failure occurred when forces and moments on the bone exceeded the strength of the weakened bone. Pseudoarthrotic features were commonly found within the components and were attributed to a 2-time fracture time line. The cause of the initial injury was suggested to be either trauma or surgically induced damage, such as excessively high implantation forces necessary to seat tight femoral prosthetic devices, leaving little or no room to express excess cement. In addition, the heat generated by the curing cement is related to the quantity of cement present under the femoral component, and an excessive amount may lead to thermal necrosis. However, the use of lesser trochanteric suction, copious pulsatile lavage, and an early reduction of the hip reduces the temperature increase to physiologic levels at the bone-cement interface as does the use of a manual bone-cement application. However, these studies have not been confirmed histologically.

( A ) Anteroposterior radiograph of the pelvis of a 63-year-old woman 1 year after receiving a BHR resurfacing device. The acetabular component is loose, and that was the reason for scheduling the revision. ( B ) Cut sections and microradiographs of the femoral head. Cement was occupying 52% and 48% (overpressurized) of the posterior ( top ) and middle sections ( bottom ), respectively. The femoral head demonstrated a previously unrecognized incomplete fracture at the junction of the deeply penetrated cement, but the head was mostly viable and vascular.

The Role of Vascularity

Little and colleagues proposed that fractures occurred because the femoral head bone was devascularized and weakened during the surgery, specifically because resurfacing was performed through the posterior approach, which is thought to destroy the extraosseous blood supply to the femoral head. This proposition was based on the histologic observation of extensive background necrosis among 12 of 13 failed resurfacings from a cohort of 377, mostly BHR, implants from the investigators’ institution. However, there are no other retrieval studies that have shown such a high proportion of necrosis, and histologic sections showing the degree of necrosis have not been published. Further, these determinations were made on a single section, which may not be representative of the entire femoral head. Blood flow in the femoral head has been studied by the same Oxford group using oxygen/nitrogen saturations and by others using laser Doppler flowmetry technique. These studies showed that dislocation and subsequent tissue dissection during the posterior approach led to a marked reduction in blood supply to the sampled areas of the femoral head. Because the volume of the bone sampled by the probe was not disclosed, it is not clear if the measurements reflect a similar loss of blood supply throughout the femoral head because of the limited areas of sampling. Khan and colleagues, using cefuroxime, a cephalosporin routinely given for antibiotic prophylaxis during hip surgery, as an indicator of the blood supply to the femoral head, reported that the posterolateral approach was associated with a significant reduction in the blood supply compared with the transgluteal approach. These studies are inconclusive in our view because the degree of avascularity has not yet been correlated with the incidence of femoral neck fractures. Recent studies on resurfaced femoral heads with positron emission tomographic (PET) scans reported good bone viability, but it is not clear if this is due to preservation of the original supply or reestablishment of temporarily compromised vascularity. Because the incidence of femoral neck fracture is very low and varies among surgeons, the significance of these studies remains controversial. In a PET scan study limited by a less number of subjects, Ullmark and colleagues found 3 cases of head necrosis at 1 year of follow-up and concluded that the delayed onset of the necrosis did not support the hypothesis of surgically damaged bone caused by avascularity. In addition, Hananouchi and colleagues showed that the intraosseous vascular network around the head-neck junction is not compromised sufficiently after resurfacing to induce complete avascularity. Because the incidence of femoral neck fracture is very low and varies among surgeons, the significance of these studies remains controversial. The amount of extreme internal rotation and duration of that position during acetabular preparation and biologic variability among patients are also possible contributing factors.

Although diminution of blood flow would be anticipated in normal, nonarthritic femoral heads after the section of the lateral circumflex artery, we have invariably observed intraoperative gross vascularity with bleeding surfaces from the arthritic head. Because some attenuation would be anticipated even in the arthritic hip, this bleeding suggests that a hypervascular state occurs within the osteoarthritic hip. This observed hyperemia has been verified with PET scan studies by Forrest and colleagues. Blood supply to the osteoarthritic femoral head must occur by anastomosis and collateral circulation through the neck to the femoral head because the osteophytes often associated with the osteoarthritic disease generally occlude the branches from the medial circumflex artery, which normally enter the head at the head-neck junction.

Extensive histologic analyses of failed resurfaced femoral heads by Campbell and colleagues have demonstrated some areas of dead bone in short-term retrievals with surrounding new bone formation, but rarely whole head necrosis. These healing areas of necrotic bone generally have ample reserve to sustain overall viability.

There is a concern that the prevalence of femoral neck fractures in women may increase with advancing age as compared with that in men. Hip resurfacing has been shown to preserve the bone mineral density (BMD) of the proximal femur over time after surgery, when the operated hip is compared with the contralateral hip. Also, BMD is better preserved after resurfacing than after a conventional total hip arthroplasty (THA). Age itself is not a predictor of femoral neck fracture after hip resurfacing. No evidence exists in the literature that women who underwent resurfacing sustain a nontraumatic neck fracture in their lifetime. According to the North American Menopause Society, postmenopausal women who are older than 65 years or weigh less than 57.73 kg (127 lb) are most at risk for osteoporosis. Ahlborg and colleagues showed that postmenopausal women experience an increase in skeletal size as a result of periosteal apposition, which at least partially compensates for the potential decrease in bone strength from the loss of bone density.

Based on the literature, certain physiologic changes that occur naturally with aging can in fact provide positive bone modifications that may reduce the risk of fracture and benefit the outcome of the prosthesis. Further follow-up is necessary to determine whether our belief that well-performed hip resurfacings in women with osteoarthritis are not vulnerable to subsequent fracture with advancing age.

In summary, it seems that most causes of femoral fractures are multifactorial and usually related to deficiencies in technique, whereas the significance of vascular changes remains controversial.

Prevention of Femoral Neck Fractures

Most surgeons continue to use the posterior approach because it is more direct, and optimal component orientation is facilitated. The advantages of other approaches introduced mostly to preserve the blood supply have not been conclusively demonstrated. Notches should be avoided, and the component needs to be fully seated. Performing cylindrical reaming in stages with a system allowing the pin to be moved up until the last ream will prevent any risk of notching. The preservation of the anterior osteophyte, when present, has been suggested because this osteophyte may play a supporting role in the overall strength of the neck. This preservation can be achieved by orienting the femoral component more anteriorly on the neck and inserting it slightly posterior to anterior. Several surgeons have proposed other approaches to minimize the potential risk of loss of blood supply: the Ganz approach with a trochanteric flip or an anterior approach. Although the exposure is excellent with the trochanteric flip, there has been a high incidence of trochanteric nonunion (10%) often associated with local bursitis, requiring removal of the screws. The exposure obtained with the anterior approach, we believe, is more limited and may not be suitable for all patients. However, it is generally recommended that surgeons learning the resurfacing technique should use the approach with which they are most familiar.

Femoral component alignment may be a factor in the prevention of femoral neck fractures, and the use of navigation has been recommended as a surgical tool to optimize orientation.

Several investigators recommend avoiding high-load physical activities during the time needed for the bone to remodel around the prosthesis, but this recommendation also warrants further investigation.

Incidence of Femoral Neck Fractures

To assess the worldwide incidence of femoral neck fracture after modern hip resurfacing, we collected data from the reports providing this information since 2005 and selected the most recent when several reports were published based on the same series of patients. Our computations yielded a 1.69% incidence of femoral neck fracture (range, 0%–9.2%) for a global cohort of 10,381 cases, including 3497 from the Australian Hip Registry ( Table 1 ).

Etiology of Femoral Neck Fractures

The variability reflects the influence of surgeon experience, criteria for patient selection, component design, and surgical technique. Campbell and colleagues have performed extensive studies of failed MMSA. Failures ranged from 1 week to several years postoperatively. Of the 98 failures submitted for analysis, the most common cause of failure was femoral neck fracture (32%). Most of these fractures occurred within 2 months of surgery, but in 7 cases, the average time to fracture was 15 months. The procedure of resurfacing itself is thought to increase strains in the lateral aspect of the femoral neck and the lesser trochanter region. Shimmin and Back reviewed the resurfacing experience within the Australian National Registry of joint replacements, which includes predominately Birmingham hip resurfacing (BHR) (Smith and Nephew, Inc, Memphis, TN, USA) types, and reported that fractures of the femoral neck were twice as likely in women than men and were more prevalent when the femoral implant was placed in the varus. The effect of femoral component positioning on the potential risk of neck fracture was further studied by the finite element analysis or cadaver studies, and the consensus is that valgus placement is recommended to avoid femoral neck fracture, although Vail and colleagues suggest that small deviations from the anatomic placement of the component result in high localized stresses on the femoral neck.

The multifactorial nature of this complication was first highlighted by Amstutz and colleagues, who showed that most fractures that occurred in hips had more than 1 possible cause. Fractures that occurred early, within 2 months, were located at the neck component junction (subcapital), and the fracture of the hip, which occurred later (at 20 months post operatively), was located within the femoral head (intracapital) at the junction of viable and nonviable or repairing bone.

Notching has been reported in both fractured and successful resurfacings. Occasionally, stress fractures occurring under MMRA components have been reported to heal if noted early and if the hip is protected by a period of non–weight-bearing. Bone undergoing repair following stress fracture is a site of weakness for eventual fatigue failure.

Several incomplete (premonitory) and complete subcapital fractures were found unexpectedly by Campbell and colleagues after retrieval in resurfacing revised for pain or femoral migration ( Fig. 1 ). This case also illustrates the danger of over-pressurization of cement into the dome, which has been observed more often in those devices in which there is minimal or no cement mantle. Many of these failures can be considered to have a dual-phase failure mode; the original trauma to the bone occurs at surgery, but the actual failure process takes place several weeks or months later. Histologic analysis of the short-term fractures showed that the break occurred through areas of healing cancellous and cortical bone at the component/neck region. Similar findings were reported by Morlock and colleagues after the analysis of fracture patterns and histologic findings among 141 femoral resurfacing failures. Early fractures were associated with uncovered reamed bone and microfractures, with associated healing. The investigators speculated that failure occurred when forces and moments on the bone exceeded the strength of the weakened bone. Pseudoarthrotic features were commonly found within the components and were attributed to a 2-time fracture time line. The cause of the initial injury was suggested to be either trauma or surgically induced damage, such as excessively high implantation forces necessary to seat tight femoral prosthetic devices, leaving little or no room to express excess cement. In addition, the heat generated by the curing cement is related to the quantity of cement present under the femoral component, and an excessive amount may lead to thermal necrosis. However, the use of lesser trochanteric suction, copious pulsatile lavage, and an early reduction of the hip reduces the temperature increase to physiologic levels at the bone-cement interface as does the use of a manual bone-cement application. However, these studies have not been confirmed histologically.

( A ) Anteroposterior radiograph of the pelvis of a 63-year-old woman 1 year after receiving a BHR resurfacing device. The acetabular component is loose, and that was the reason for scheduling the revision. ( B ) Cut sections and microradiographs of the femoral head. Cement was occupying 52% and 48% (overpressurized) of the posterior ( top ) and middle sections ( bottom ), respectively. The femoral head demonstrated a previously unrecognized incomplete fracture at the junction of the deeply penetrated cement, but the head was mostly viable and vascular.

The Role of Vascularity

Little and colleagues proposed that fractures occurred because the femoral head bone was devascularized and weakened during the surgery, specifically because resurfacing was performed through the posterior approach, which is thought to destroy the extraosseous blood supply to the femoral head. This proposition was based on the histologic observation of extensive background necrosis among 12 of 13 failed resurfacings from a cohort of 377, mostly BHR, implants from the investigators’ institution. However, there are no other retrieval studies that have shown such a high proportion of necrosis, and histologic sections showing the degree of necrosis have not been published. Further, these determinations were made on a single section, which may not be representative of the entire femoral head. Blood flow in the femoral head has been studied by the same Oxford group using oxygen/nitrogen saturations and by others using laser Doppler flowmetry technique. These studies showed that dislocation and subsequent tissue dissection during the posterior approach led to a marked reduction in blood supply to the sampled areas of the femoral head. Because the volume of the bone sampled by the probe was not disclosed, it is not clear if the measurements reflect a similar loss of blood supply throughout the femoral head because of the limited areas of sampling. Khan and colleagues, using cefuroxime, a cephalosporin routinely given for antibiotic prophylaxis during hip surgery, as an indicator of the blood supply to the femoral head, reported that the posterolateral approach was associated with a significant reduction in the blood supply compared with the transgluteal approach. These studies are inconclusive in our view because the degree of avascularity has not yet been correlated with the incidence of femoral neck fractures. Recent studies on resurfaced femoral heads with positron emission tomographic (PET) scans reported good bone viability, but it is not clear if this is due to preservation of the original supply or reestablishment of temporarily compromised vascularity. Because the incidence of femoral neck fracture is very low and varies among surgeons, the significance of these studies remains controversial. In a PET scan study limited by a less number of subjects, Ullmark and colleagues found 3 cases of head necrosis at 1 year of follow-up and concluded that the delayed onset of the necrosis did not support the hypothesis of surgically damaged bone caused by avascularity. In addition, Hananouchi and colleagues showed that the intraosseous vascular network around the head-neck junction is not compromised sufficiently after resurfacing to induce complete avascularity. Because the incidence of femoral neck fracture is very low and varies among surgeons, the significance of these studies remains controversial. The amount of extreme internal rotation and duration of that position during acetabular preparation and biologic variability among patients are also possible contributing factors.

Although diminution of blood flow would be anticipated in normal, nonarthritic femoral heads after the section of the lateral circumflex artery, we have invariably observed intraoperative gross vascularity with bleeding surfaces from the arthritic head. Because some attenuation would be anticipated even in the arthritic hip, this bleeding suggests that a hypervascular state occurs within the osteoarthritic hip. This observed hyperemia has been verified with PET scan studies by Forrest and colleagues. Blood supply to the osteoarthritic femoral head must occur by anastomosis and collateral circulation through the neck to the femoral head because the osteophytes often associated with the osteoarthritic disease generally occlude the branches from the medial circumflex artery, which normally enter the head at the head-neck junction.

Extensive histologic analyses of failed resurfaced femoral heads by Campbell and colleagues have demonstrated some areas of dead bone in short-term retrievals with surrounding new bone formation, but rarely whole head necrosis. These healing areas of necrotic bone generally have ample reserve to sustain overall viability.

There is a concern that the prevalence of femoral neck fractures in women may increase with advancing age as compared with that in men. Hip resurfacing has been shown to preserve the bone mineral density (BMD) of the proximal femur over time after surgery, when the operated hip is compared with the contralateral hip. Also, BMD is better preserved after resurfacing than after a conventional total hip arthroplasty (THA). Age itself is not a predictor of femoral neck fracture after hip resurfacing. No evidence exists in the literature that women who underwent resurfacing sustain a nontraumatic neck fracture in their lifetime. According to the North American Menopause Society, postmenopausal women who are older than 65 years or weigh less than 57.73 kg (127 lb) are most at risk for osteoporosis. Ahlborg and colleagues showed that postmenopausal women experience an increase in skeletal size as a result of periosteal apposition, which at least partially compensates for the potential decrease in bone strength from the loss of bone density.

Based on the literature, certain physiologic changes that occur naturally with aging can in fact provide positive bone modifications that may reduce the risk of fracture and benefit the outcome of the prosthesis. Further follow-up is necessary to determine whether our belief that well-performed hip resurfacings in women with osteoarthritis are not vulnerable to subsequent fracture with advancing age.

In summary, it seems that most causes of femoral fractures are multifactorial and usually related to deficiencies in technique, whereas the significance of vascular changes remains controversial.

Prevention of Femoral Neck Fractures

Most surgeons continue to use the posterior approach because it is more direct, and optimal component orientation is facilitated. The advantages of other approaches introduced mostly to preserve the blood supply have not been conclusively demonstrated. Notches should be avoided, and the component needs to be fully seated. Performing cylindrical reaming in stages with a system allowing the pin to be moved up until the last ream will prevent any risk of notching. The preservation of the anterior osteophyte, when present, has been suggested because this osteophyte may play a supporting role in the overall strength of the neck. This preservation can be achieved by orienting the femoral component more anteriorly on the neck and inserting it slightly posterior to anterior. Several surgeons have proposed other approaches to minimize the potential risk of loss of blood supply: the Ganz approach with a trochanteric flip or an anterior approach. Although the exposure is excellent with the trochanteric flip, there has been a high incidence of trochanteric nonunion (10%) often associated with local bursitis, requiring removal of the screws. The exposure obtained with the anterior approach, we believe, is more limited and may not be suitable for all patients. However, it is generally recommended that surgeons learning the resurfacing technique should use the approach with which they are most familiar.

Femoral component alignment may be a factor in the prevention of femoral neck fractures, and the use of navigation has been recommended as a surgical tool to optimize orientation.

Several investigators recommend avoiding high-load physical activities during the time needed for the bone to remodel around the prosthesis, but this recommendation also warrants further investigation.

Incidence of Femoral Component Loosening

The incidence of femoral loosening in most reported series with a 5-year follow-up is low, ranging from 0% to 1.3%. However, these series were essentially composed of patients with predominately large component sizes and small cystic defects. A 2% rate of femoral loosening was reported in a series of all comers with a mean follow-up of 5.6 years.

A higher incidence of femoral component loosening (5 of 104) has been reported at a mean follow-up of 4.3 years in a series of hips with osteonecrosis. In this report and 2 other series, the surgeons accepted no more than 30% of necrotic involvement in the femoral head; Revell and colleagues reported 97.3% femoral component survival at 6.1 years, and Mont and colleagues mentioned 1 femoral loosening in 42 hips at a mean follow-up of 41 months. In contrast, despite an absence of restrictions on the degree of necrotic involvement acceptable in the femoral head, a femoral survivorship of 97% at 10 years was recently reported, with only 2 cases of femoral loosening in 85 hips and none in surgeries performed after 1997. These rather remarkable results are believed to be the result of a thorough removal of all the yellow necrotic bone with a high-speed burr in hips with osteonecrosis, combined with bone cleansing, drying, and optimized cementing technique. As the results of longer follow-ups become available on the series of patients with hip osteonecrosis, it becomes apparent that the indications for metal-on-metal resurfacing are expanding because it provides a much more reliable pain relief than hemi-resurfacing. To obviate femoral loosening, Gross and Back reported promising midterm results in a small group of patients with an uncemented femoral resurfacing component, although their cohort still experienced only a 78.9% survival rate at 7 years because of other causes, including 2 cases of acetabular loosening. Also, Lilikakis and colleagues showed early success with the same cementless design in a series of 66 patients (70 hips).

Etiology of Femoral Component Loosening

The main risk factors for femoral loosening are small component size and femoral head defects larger than 1 cm ( Fig. 2 ), identified in a 2004 publication of the results for 400 resurfaced hips, and a low body mass index (BMI). Aseptic loosening of the femoral component is a time-dependant process, for which an early detection can be helpful. Several methods have been proposed, including measurement of plain radiographs, Einzel-Bild-Röntgen-analyze (EBRA), and radiostereophotogrammetry (RSA). However, none of these methods provide the means to reliably study component migration on a large series. RSA is not practical to use in a routine clinical setting, and EBRA or plain radiographic measurement is reliable only if the radiographs compared present the same femoral rotation (which is rarely the case in any series of radiographs), because they are 2-dimensional solutions measuring a 3-dimensional phenomenon.

( A ) Anteroposterior radiograph of the pelvis of a 51-year-old woman with osteoarthritis. Note the extensive femoral head defects left after removal of the cystic material. Not all of the cystic material was removed from the head, and there was less-than-optimal cleaning and drying, leading to less-than-optimal initial fixation. The stem was also not cemented in, as would be our practice today with cysts of this size and small component size. ( B ) One hundred and eleven months after resurfacing. The 38-mm femoral component is loose, as shown by the large radiolucency that was first observed around the metaphyseal stem at 12 months. The well-fixed acetabular component (abduction 43° and anteversion 12.6°) was retained at revision surgery and matched with a unipolar head of appropriate diameter. (Her serial ion studies were consistently normal for a bilateral resurfacing, with cobalt, 3.4 ppb, and chromium, 7.1 ppb, at 76 months postoperation.)

Other investigators have suggested that the inner geometry of the femoral component was associated with potential debonding and stress shielding or stress concentration, which might adversely affect survivorship. Although legitimate in theory, these claims remain to be confirmed by the rapid fall of survivorship in a large series of MMRA, and no such report has been published so far.

A fibrous interface is to be avoided because movement between the bone and the cement cannot be tolerated. Retrieval analyses of long-term resurfacings show variable amounts of bone remodeling. Areas of bone thinning are often accompanied by solidification of the bone elsewhere, which can provide remarkable durability despite major bone loss. This process highlights the remarkable adaptive capability of the human femoral head to withstand the damage induced by resurfacing.

Prevention of Femoral Component Loosening

Amstutz and colleagues showed the effectiveness of modified surgical techniques on the rate of femoral component loosening, especially in patients with risk factors. A review of the intraoperative photos of the prepared femoral head in conjunction with a component retrieval analysis led to substantial improvements in component fixation techniques ( Fig. 3 ).

( A ) Anteroposterior radiograph of the pelvis of a 54-year-old man with bilateral osteoarthritis. The inset shows the femoral head after preparation (second generation). Note the complete removal of all cystic material, how clean and dry the bone is, and the abundance of drill holes in the chamber and dome portions of the head, particularly in sclerotic bone. ( B ) Seven years after left staged hip resurfacing and 5 years after right staged hip resurfacing, the patient’s University of California at Los Angeles hip scores are 8 ( left ) and 10 ( right ) for pain, 10 for walking, 10 for function, and 7 for activity. Note the absence of heterotopic bone formation on the right hip, which was treated with our radiotherapy protocol in contrast to the left hip for which only indomethacin was used.

The evolution of the surgical technique and its effects on prosthetic survival have previously been published and follow 4 principles:

• 1.
  

  Thorough cleaning of the femoral head. This includes the removal of any cystic material with a high-speed burr and cleansing with lavage.

  
  
• 2.
  

  Maximization of the area for fixation. This includes the use of multiple small drill holes performed with a one-eighth-of-an-inch drill bit, rather than fewer larger holes.

  
  
• 3.
  

  Drying of the femoral head before cementation. This includes the use of suction to keep the bone dry until the acrylic has set and is also facilitated by the use of CO ₂ applied with the Carbojet (Kinamed Inc, Camarillo, CA, USA) as shown in Fig. 3 A.

  
  
• 4.
  

  Cementation with a uniform cement mantle of about 1 mm and a penetration of 1 to 3 mm. A uniform pressurization is achievable with regular acrylic in a doughy stage rather than top-down pressurization of low-viscosity cement, which leads to overpressurization of the dome. Timing is critical, and the 1-mm gap allows extrusion of excess cement without overpenetrating the proximal bone (see Fig. 1 ).

In addition, the effectiveness of cementing the metaphyseal stem for small component sizes and hips with large defects was recently demonstrated. The cementation of the femoral stem adds surface area for fixation, although it could potentially increase stress shielding within the femoral head. However, clinical results of hips implanted with this technique currently show an absence of femoral component loosening. In this study, caution about the practice of impact sports was raised for patients with a cemented metaphyseal stem because cement has a different modulus of elasticity than the bone, and repetitive impact may initiate micromotion between bone and cement and contribute to a loosening process.

Incidence of Acetabular Component Loosening

All currently available hip resurfacing systems use a cementless acetabular component. The superiority of cementless acetabular fixation for MMRA was clearly demonstrated by Beaulé and colleagues. The reported incidence of acetabular component loosening is low for most cementless designs. At a mean follow-up of 5 years, Hing and colleagues reported 1 (0.4%) aseptic acetabular component loosening from a series of 230 BHR resurfacings. O’Neill and colleagues reported 2 (0.8%) cases of acetabular component loosening in a series of 250 hips resurfaced by 5 surgeons using a variety of modern-generation resurfacing devices, and Amstutz and Le Duff reported no acetabular component loosening in 1000 hips at a mean follow-up of 5.6 years (range, 1–11 years). This issue contains the first report of radiographic studies specifically addressing the performance of 1-piece metal-on-metal acetabular components used in hip resurfacing, with a 5-year minimum follow-up (see the article by Ball and colleagues elsewhere in this issue for further exploration of this topic). Using radiographic evidence of loss of acetabular fixation (complete radiolucency covering all 3 zones), this study reported a Kaplan-Meier survival estimate of the acetabular component of 99.6% at 5 years and 97.6% at 10 years. Kim and colleagues reported 10 (5%) cases of acetabular loosening in a multicenter series of 200 hips and attributed this result to the surgeons’ learning curve.

In contrast, several reports raise concern relating a high incidence of acetabular loosening with 2 designs. Cutts and colleagues used the Cormet device (Corin Medical Ltd, Cirencester, UK) and reported 4 cases of acetabular component loosening in 65 hips (6.2%). This report was corroborated by Dixon and colleagues, who reported a 10% aseptic loosening rate at 5 years with the same design. Long and colleagues reported a high short-term failure rate (mean, 1.6 years) in 29 of 207(14%) hips implanted with Durom monoblock cups (Zimmer, Warsaw, IN, USA) used with conventional stem-type THA, although a group of surgeons from Montreal had better results in the use of this acetabular component for resurfacing. This device has now been withdrawn from the US market by the manufacturer. At the very least, it seems that the surgical technique for this system was critical and required that the acetabular preparation be sufficiently deep to completely seat the component within the acetabular walls.

Historically, a higher rate of acetabular component loosening has been reported in patients with developmental dysplasia of the hip (DDH) treated with hip resurfacing. Several reports show that the current acetabular component designs perform well in this challenging etiologic group, whether adjunct fixation is used or not. However, 1 center found a greater rate of acetabular component loosening in patients with DDH compared with patients with primary osteoarthritis.

Prevention of Acetabular Component Loosening

Surgical technique is key to both the initial and enduring fixation of the acetabular component. The facility to achieve initial fixation varies with the different designs that have a variable amount of hemispherical coverage (165°–180°), roughness of coatings (porous beads of various sizes, plasma spray), and different methods of preparation and insertion (instruments and techniques) as shown in Table 2 .

Table 2

Technical characteristics of the acetabular components for the currently used hip resurfacing designs

Bearing					Acetabular Component
System	Date	Process	Heat Treatment	Clearance (μ)	Coverage (°)	Shape	Surface	Shell Thickness (mm)	Instrumentation
Conserve® Plus	1996	Wrought or cast (Fem) Cast (Acet)	HIP, SA	100–220	170 outside, 170 inside, constant for all sizes	Truncated hemisphere	Sintered Co-Cr beads (0.3-mm diameter; 0.75-mm thickness) ± HA	3.5 or 5.5	Rigid bayonet coupling; inserter removable; capacity to reattach, extract, and reimplant component
BHR	1997	Cast	None	260 (size 54)	180 outside, 164 inside, decreasing with smaller sizes	Truncated hemisphere	Co-Cr beads (0.9–1.3 mm) cast-in + HA	3 rim, 6 dome	Instrument for insertion attached by wires, and once cut, extraction no longer possible
Cormet 2000	1997	Cast	HIP, SA	150–400 a	—	Equatorial expansion	Ti-VPS + HA	3 and 4 (2 cups per head)	Inserter fits into cup face recesses, no extractor
Durom	2001	Wrought/forged	None	150 range unknown	165 inside, constant for all sizes	Truncated hemisphere	Ti-VPS	4	Inserter fits into cup face recesses, no extractor
ASR	2003	Cast	HIP, SA	100 midrange size b	170 outside, 156 inside, decreasing with smaller sizes	Truncated hemisphere	Sintered Co-Cr beads + HA	3–5	Inserter fits in recesses inside the socket, reducing head coverage
ReCap (Biomet)	2004	Cast	None	240 for size 46 c	180 outside, inside unknown	Hemisphere	Ti-VPS ± HA	4	Equatorial diameter 2 mm greater than dome; inserter fits into cup face recesses, no extractor

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related

Related posts:

Comparison of Functional Results of Hip Resurfacing and Total Hip Replacement: A Review of the Literature Imaging of Metal-On-Metal Hip Resurfacing A Prospective Metal Ion Study of Large-Head Metal-on-Metal Bearing: A Matched-Pair Analysis of Hip Resurfacing Versus Total Hip Replacement Complications After Metal-on-Metal Hip Resurfacing Arthroplasty The Future of Hip Resurfacing A Prospective Metal Ion Study of Large-Head Metal-on-Metal Bearing: A Matched-Pair Analysis of Hip Resurfacing Versus Total Hip Replacement

Stay updated, free articles. Join our Telegram channel

Join