Orthopedics

Feature Article 

Preservation of Bone Mineral Density of the Proximal Femur Following Hemisurface Arthroplasty

Harlan C Amstutz, MD; Edward Ebramzadeh, PHD; Akos Sarkany, MD; Michel Le Duff, MA; Robert Rude, MD

Abstract

abstract

Bone mineral density of the proximal femur was measured in six patients who underwent hemisurface replacement for osteonecrosis of the femoral head, Bone mineral density values in operated and contralateral nonoperated hips were compared. In four patients who had sequential examinations, bone mineral density was compared overtime. Average patient age was 34.6 years, average follow-up was 9.1 years, and mean follow-up of bone mineral density measurements was 6.6 years. Average bone mineral density variation was 0.0048 to -0.0264 g/cma per year in all five regions in nonoperated hips and -0.012 to -0.0300 g/cm2 in operated hips. These results support bone conservation and preservation with hemi resurfacing arthroplasty in young patients with osteonecrosis of the femoral head.

Abstract

abstract

Bone mineral density of the proximal femur was measured in six patients who underwent hemisurface replacement for osteonecrosis of the femoral head, Bone mineral density values in operated and contralateral nonoperated hips were compared. In four patients who had sequential examinations, bone mineral density was compared overtime. Average patient age was 34.6 years, average follow-up was 9.1 years, and mean follow-up of bone mineral density measurements was 6.6 years. Average bone mineral density variation was 0.0048 to -0.0264 g/cma per year in all five regions in nonoperated hips and -0.012 to -0.0300 g/cm2 in operated hips. These results support bone conservation and preservation with hemi resurfacing arthroplasty in young patients with osteonecrosis of the femoral head.

Treatment of osteonecrosis of the femoral head remains a challenge -A. for orthopedic surgeons. Patients with osteonecrosis tend to be young and active. Osteonecrosis can be secondary to a systemic disease affecting multiple joints and organ systems, part of the sequelae of injury, or ìdìopathic (Figure IA). For nontraumatic osteonecrosis of the hip, bilateral involvement is common, occurring in 50% to 80% of all cases.1'5 Surgical treatment includes coring (decompression with or without bone grafting), sometimes combined with electrical stimulation, vascularized fibular grafting, derotational osteotomies, hemiarthroplasty, and total hip arthroplasty (THA).

Total hip arthroplasty, when performed in young, active patients with osteonecrosis of the femoral head, generally has a high revision rate.1·6"8 Stemtype THA prostheses also are associated with proximal femoral bone loss, as demonstrated by sequential bone mineral density measurements. il The iatrogenic femoral bone loss following conventional THA may make revisions more difficult and complicated.

An alternative treatment for Ficat stage Il to early stage IV osteonecrosis of the femoral head in young, active patients is hemisurface arthroplasty (Figure IB).12"16 Hemisurface arthroplasties are intended to reduce initial and subsequent bone loss compared to stemmed hemiarthroplasty devices because the femoral head and neck are preserved, thereby allowing a more physiological mechanism of load transfer, and because polyethylene wear debris has not been observed after the implantation of bipolar devices.17

This study assessed bone remodeling in the femur of osteonecrosis patients after hemisurface replacement and evaluated bone remodeling changes over time. This is the first reported study that has measured bone mineral density in patients who have undergone hemisurface arthroplasties. The results are valuable in determining the prognosis of patients with osteonecrosis treated with surface hemiarthroplasty. Furthermore, because of the similarities in load transfer mechanism to the resurfaced femoral head and neck, and because of the absence of polyethylene wear debris, this study may be useful to infer the safety and efficacy of primary metal-on-metal surface replacements.

Figure 1: Preoperative radiograph of a 31-year-old woman (patient 1) with early Ficai stage IV osteonecrosis secondary to steroid-induced osteonecrosis and systemic lupus erythematosus (A). Postoperative radiograph taken after 18 years shows well-preserved bone structure with no neck narrowing and new bone formation In the acetabular fossa, characteristic of all long-term follow-up (B). Although the joint space is narrow, the patient remains relatively asymptomatic. Rflure 2: Regions of interest indicated on dual-energy x-ray absorptiometry densitometry Images of a 39-year-old man (patient 5) with trauma-induced osteonecrosis 26 months postsurgery. Both the operated side, with hemisurface replacement prosthesis, and the contralateral nonoperated side are shown.

Figure 1: Preoperative radiograph of a 31-year-old woman (patient 1) with early Ficai stage IV osteonecrosis secondary to steroid-induced osteonecrosis and systemic lupus erythematosus (A). Postoperative radiograph taken after 18 years shows well-preserved bone structure with no neck narrowing and new bone formation In the acetabular fossa, characteristic of all long-term follow-up (B). Although the joint space is narrow, the patient remains relatively asymptomatic. Rflure 2: Regions of interest indicated on dual-energy x-ray absorptiometry densitometry Images of a 39-year-old man (patient 5) with trauma-induced osteonecrosis 26 months postsurgery. Both the operated side, with hemisurface replacement prosthesis, and the contralateral nonoperated side are shown.

MATERIALS AND METHODS

Twenty patients with Ficat stage ?? or early stage IV osteonecrosis, as determined by radiograpMc analysis, underwent hemiresurfacing between 1981 and 1994. Ten patients who had bilateral disease (Charnley class B and C), 3 unilateral patients who were lost to follow-up, and 1 patient with postoperative heterotopic ossification were excluded. Of the 10 patients with unilateral disease (Charnley class A), 6 (2 women and 4 men) met the inclusion criteria for the study.

All six patients had Ficat stage ?? disease. Four patients had posttraumatic osteonecrosis, one had steroid-induced osteonecrosis, and one case was idiopathic. Average patient age was 34.6 years (range; 18-50 years), and average patient weight was 82.6 kg (range: 50.5-120.3 kg). Average follow-up time was 1 10 months (range: 54-223 months).

Hemiresurfacing was performed with cemented ceramic (four hips), titanium alloy (one hip), or cobalt-chrome alloy (one hip) components. Five patients had trochanteric osteotomies, all of which healed uneventfully. Two patients had single bone mineral density measurements, and four had serial bone mineral density measurements. Radiographie appearance of the neck of both the operated and nonoperated sides was observed, and neck width was measured postoperatively and at final follow-up.

Dual-energy x-ray absorptiometry (DXA) was used to quantify periprosthetic bone mineral density of the subcapital area, the neck, and the proximal femur. Bone mineral density was measured using a Hologic QDR-2000 densitometer and Orthopaedic software (Hologic Ine, Waltham, Mass). All measurements were taken by the same operator, with patients positioned with both legs intemaily rotated 15°.

Figure 3: Graph depicting average bone mineral density values (g/cm2) of the operated and nonoperated proximal femur in patients with unilateral hemisurface arthroplasty. Abbreviations: Non-op=nonoperated and Op=operated.

Figure 3: Graph depicting average bone mineral density values (g/cm2) of the operated and nonoperated proximal femur in patients with unilateral hemisurface arthroplasty. Abbreviations: Non-op=nonoperated and Op=operated.

In addition to global determinations, five regions of interest were selected for analysis:

* R1-femoral neck,

* R2-intertrochanteric area,

* R3-lesser trochanter,

* R4-trochanter, and

* RS-subtrochanteric area (Figure 2).

Bone mineral density was compared in the operated and contralateral nonoperated hips for each region. Moreover, for patients who had sequential examinations, bone mineral density in each region of the first scan was compared to that of the second scan for both operated and nonoperated sides.

Mean bone mineral density values of the operated and nonoperated sides were analyzed and compared using Student's / test. Linear regression was used to determine the overall change in bone mineral density over time.

RESULTS

Average time from surgery to bone mineral density measurement was 79 months (range: 4-223 months). Bone mineral density results of individual patients in each region are shown in Table 1. As expected, average bone mineral density was somewhat lower in hips with hemiresurfacing compared to nonoperated sides; however, the differences between operated and nonoperated sides were small (Figure 3).

Results from the four patients with serial densitometries demonstrated small changes over time for both operated and nonoperated sides. The interval between these measurements ranged from 38 to 80 months.

The UCLA ??-point rating scale, with one indicating the greatest level of disability, was used to assess pain, walking, function, and activity of the patients. Average preoperative scores for the six patients included in the study were 4.3 for pain, 6.0 for walking, 5.3 for function, and 4.1 for activity. At patients' last clinical evaluation, average scores were 8.5 for pain, 8.8 for waUdog, 8.8 for function, and 6.3 for activity, reflecting significant clinical improvement following hemiresurfacing. Average postoperative Harris hip score (HSS) was 88.8 (preoperative scores were not available). One patient underwent revision and conversion to THA at 112 months after hemiresurfacing surgery because of hip pain secondary to acetabular cartilage wear.

Table

TABLE 1Average and Global Bone Mineral Density Values of the Operated and Nonoperated Proximal Femur in Hemiarthroplasty Patients

TABLE 1

Average and Global Bone Mineral Density Values of the Operated and Nonoperated Proximal Femur in Hemiarthroplasty Patients

Radiographie analysis demonstrated well-united and well-healed trochanteric osteotomies. Radiographically, bone stock was preserved bilaterally. The neck region did not show any osteolysis or narrowing, and there was no evidence of prosthetic loosening.

On anteroposterior radiographs, neck width of the nonoperated side was measured at patients* last follow-up visit and compared to the operated side. No difference in width between the two sides was found. Gross examination and sections from a retrieved hemisurface implant from one of the patients who underwent revision surgery after 10 years revealed excellent bone stock (Figure 4).

Average bone mineral density of the nonoperated hips varied between 0.0048 and -0.0264 g/cm2 per year in all five regions (Table 2). Bone mineral density of the operated side also decreased by similar rates, between -0.012 and -0.0300 g/cm2 per year (Figure 5).

Table

TABLE 2Results of Linear Regression Analysis for Changes in Bone Mineral Density Over Time for Operated and Nonoperated Hips*

TABLE 2

Results of Linear Regression Analysis for Changes in Bone Mineral Density Over Time for Operated and Nonoperated Hips*

DISCUSSION

This study indicates that with hemiresurfacing, bone is conserved and preserved in both the neck and proximal femoral shaft. There was no evidence of any adverse femoral neck bone remodeling. The slight bilateral decrease in bone mineral density found in this study was consistent with mat of the aging process.18

However, two patients exhibited increases in BMD in zone 1 over time. One patient showed a 14% increase on the nonoperated side, which possibly can be explained by a 17% increase in body weight during that period.18'21 A second patient showed a 9% increase on the operated side, most likely due to the first DXA being performed only 4 months postsurgery when the patient had not yet resumed a normal activity level.20,22

In contrast to hemisurfacing, with conventional THA, die neck is resected, resulting in stress shielding in the proximal femur and progressive decline in bone mineral density.9,11,23,24 With conventional THA, bone loss occurs more proximally, ranging from 25.5% to 57% at 2 to 3 years after surgery, depending on the type of the implant.25'29 Although most of the loss occurs during the first 3 years, Mcarthy et al" reported bone loss up to 50% at 7 to 14 years after THA.

Newer designs may provide better proximal load transfer and minimize the loss of proximal bone, but the reported findings support the need for a bone stock preserving procedure such as hemisurface arthroplasty, especially in young patients with femoral head necrosis or other hip disorders that require arthroplasty. In addition to preserving bone stock, the natural anatomy, joint biomechanics, and stability may be better preserved in surface arthroplasty. If the device should fail, more normal bone stock is available, thus facilitating revision surgery when required.

Rgure 4: Photograph of a titanium alloy hemisurface arthroplasty specimen from a 34-year-old man with steroid-induced osteonecrosis revised after 10 years demonstrates the preservation of bone stock and burnishing of the soft titanium alloy surface, presumably caused by the subchondral bone when the articular cartilage wore out (A). Photographs of 5-mm slab sections from the retrieved implant show good maintenance of trabeeular architecture throughout; small gaps at the bone-cement interface are artifacts and were produced by the sectioning process (B).

Rgure 4: Photograph of a titanium alloy hemisurface arthroplasty specimen from a 34-year-old man with steroid-induced osteonecrosis revised after 10 years demonstrates the preservation of bone stock and burnishing of the soft titanium alloy surface, presumably caused by the subchondral bone when the articular cartilage wore out (A). Photographs of 5-mm slab sections from the retrieved implant show good maintenance of trabeeular architecture throughout; small gaps at the bone-cement interface are artifacts and were produced by the sectioning process (B).

The decrease of bone mass over time in patients with hemiresurfacing measured in this study was smaller than that reported with conventional THA. This suggests conversion to full-surface replacement or THA is feasible without penalty when the articular cartilage wears out in these young patients.

The primary cause of failure of conventional THA is aseptic loosening, which, in turn, is affected by résorption of the proximal femoral bone. Bone résorption can be caused by inflammatory response against foreign-body paniculate debris or stress shielding. Decrease of bone mass in stemtype prostheses is correlated with decreased bone mineral density.9'11,23,25,27-32 Compared with primary THA, the results of revision surgery are not as favorable and are less durable, particularly in patients with diminished bone stock.

The failure rate of THA in young patients with osteonecrosis is greater than in patients with other etiologies.1,33,34 Cornell et al7 attributed failure of THA in osteonecrotic patients to poor bone quality. However, the long-term results of cemented or noncemented femoral stems in young, active, or heavy patients are not as consistently durable as desired. In one series, the revision rate of cemented THA in patients aged <30 years was 82% after an average of 16.2 years of follow-up, and in patients who underwent surgery at ages 30 to 45, the revision rate was 56%. The poorest clinical results were in patients with osteonecrosis, regardless of age.34 Blinker et aIB reported a 24.1% revision rate after an average follow-up of 68 months with noncemented primary THA in patients with osteonecrosis who were younger than 35.

Figure 5: Graphs depicting bone mineral density linear regression analysis for each region of interest for operated and nonoperated hips.

Figure 5: Graphs depicting bone mineral density linear regression analysis for each region of interest for operated and nonoperated hips.

Despite newer devices and the fact that bearing materials continue to improve, young and active patients with THA may require one or more revisions during their lifetime, with the potential for decreasing bone stock and function. Implants that preserve bone stock and allow easy revision would be of utmost benefit for young patients. Therefore, selecting a treatment option that will minimize bone loss is prudent.

With hemiresurfacing, the amount of femoral bone removed is limited to the surface of the head, removing only the diseased bone. That is, the femoral neck and femoral canal remain intact, while in contrast, with THA, the neck is sacrificed, the femoral canal is violated, and more bone is removed.

In addition, with surface arthroplasty, load transfer more closely resembles that of the natural femur than with THA. With conventional stem-type devices, there is an increased load transferred distally around the stem and decreased load transferred proximally, leading to increased loss of proximal femoral bone.21,28

Hemisurface arthroplasty has been successful in providing functional hip for most patients with Ficat stage UI and early stage- IV osteonecrosis.13,15,17,35 The results of this study, along with earlier studies comparing prosthesis durability, suggest hemisurface arthroplasty is a viable conservative treatment for osteonecrosis in young patients.

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TABLE 1

Average and Global Bone Mineral Density Values of the Operated and Nonoperated Proximal Femur in Hemiarthroplasty Patients

TABLE 2

Results of Linear Regression Analysis for Changes in Bone Mineral Density Over Time for Operated and Nonoperated Hips*

10.3928/0147-7447-20041201-16

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