Scientific Review

BONE MINERAL DENSITY AND THE ELDERLY: HIGHLIGHTS FROM THE AAOS 75TH ANNUAL MEETING SAN FRANCISCO, CA

AMY HOANG-KIM
ISFR RESEARCH COORDINATOR

IMAGING ISSUES

El-Zaim, H (Kingwood, Tx) and co-authors assessed whether standard femoral radiographs can be used to reliably assess osteoporosis (AAOS Paper No. 583). Twenty-eight cadaveric femurs were obtained from female cadaveric donors ranging in age from 49 to 98 years. Standard AP and lateral radiographs were taken of each specimen. The severity of osteoporosis was then measured using dual energy x-ray absorptiometry (DXA) and a T-score was recorded for the femur. The degree of osteoporosis of each femur was also predicted on the basis of (i) the thickness of the anterior cortex; (ii) the percentage of the width of the diaphysis occupied by cortical bone (The Cortical Index), (iii) the Dorr classification (Type A, B, C) based on the shape of the medullary canal and the posterior cortex. This data was used to calculate the sensitivity and specificity of each radiographic method. Based on the DXA values, the 28 femora ranged from normal (1), to osteopenic (6), osteoporotic (12), and severely osteoporotic (9). Using the three different parameters, the femora was correctly classified in 81% of cases using the Cortical index, 70% based on the medial cortical thickness and 67% based on the Dorr classification. The sensitivity and specificity of the three methods in detecting osteoporotic cases were: Cortical Index 95%-100%; medial cortical thickness: 86-50% and Dorr classification 81-67%. Critical cut-off values for the Cortical Index were 0.44 for osteopenia/osteoporosis, and 0.33 for mild/severe osteoporosis. Of the parameters examined, the Cortical index provides the greatest sensitivity, specificity, and accuracy, and is readily applied to plain AP radiographs.

BONE REMODELING IN THR

Bone remodeling of a metaphyseal fixed femoral stem, Mayo Conservative Hip, was characterized by DXA and a mathematical remodeling model. Chen H (Taipei, Taiwan) and co-authors included 29 patients undergoing total hip replacement (THR) with a mean age of 50.8 years and the mean follow-up of 5.7 years (ORS Poster 1). The bone mineral density of the periprosthetic femur and contralateral femur was evaluated in the anterior-posterior plane. The mathematical model was based on the assumption that the bone is a biological optimized material which tends to maximize its resistance with a minimum of structural consumption. As anticipated, bony structure underwent significant remodeling after the short-stemmed femoral arthroplasty with resorption proximally and in growth distally/laterally. The von Mises stress distribution indicated that stress was more concentrated around the distal lateral canal and the medial cortex and was in good agreement with the bone mineral distribution. The overall resorption, however, was considerably less than that in the traditional long-stemmed prosthesis. Theoretical prediction suggested that the remodeling was largely regulated by the mechanical loading distribution pattern determined by the implant design.

In a scientific exhibit on the recovery of bone mineral density in the proximal femur by Moroni A (Bologna, Italy) and co-authors, they demonstrated that significant bone loss in the proximal femur occurs during the first 3 months (AAOS Scientific Exhibit 7). The largest bone loss occurred in ROIs-1, 2, 6, 7 and 8. The reduction in joint function which occurs during the first week after the operation can explain this result. At 9 months, almost complete recovery of BMD was observed in ROIs-2, 6, 7 and 8 over time. At 24 months the recovery was such that BMD surpassed the baseline. These results showed that the metal hip resurfacing causes no post-operative bone loss when function is fully recovered. Compared to pre-op, the higher post-op BMD is a sign of improved hip function.

SURGICAL TREATMENT IN THE ELDERLY

There is much debate regarding the optimal treatment of displaced unstable distal radius fractures in the elderly. Egol K (New York, NY) and co-authors conducted a retrospective review of prospectively collected data in order to compare outcomes between elderly patients who sustained displaced distal radius fractures that were treated with or without surgical intervention (AAOS paper 399). Over a three-year period 358 patients were included. All the fractures underwent closed reductions and splinting. Patients who failed an acceptable closed reduction were offered surgical intervention. Patients who refused surgery were treated to completion with casting. Patients who underwent surgery were treated with either plate and screw fixation or external fixation. Fifty-eight percent of the patients included were above the age of 65 years. The authors concluded that there were no difference with respect to baseline demographics between the cohorts. At one year, patients who underwent surgery had better wrist extension (590 vs 470, p = 0.002), wrist flexion (530 vs 460, p=0.05) and radial deviation (300 vs 170, p<0.0001). Patients treated without surgery had lower pain scores (p=0.004). There were no differences in functional status between the two groups based on the DASH scores (p=0.46). Radiographic outcome was superior with surgical intervention. There was no difference between the groups with regard to complications. These results suggest that limitations in range of wrist motion seen with non-operative care do not seem to limit functional recovery at one year.

Czerwein J (Bronx, NY) and co-authors determined if the results of elective surgery on the cervical or lumbar spine improved quality of life issues in a population of 80 years and older (AAOS poster 366). The patient sample consisted of 27 females and 18 males with a mean age of 83 years and a mean follow-up of 28 months. Lumbar stenosis patients were treated with either posterior decompression and fusion at all levels decompressed. The mean levels decompressed was 3 in this group of 33 patients. Cervical myelopaths were treated with either anterior (4 patients) or posterior (9 patients) decompression and fusion at all levels decompressed. The mean levels decompressed was 4 in this group of 12 patients.Lumbar stenosis patients showed a visual analog scale improvement of 62% in leg pain and 54% in back pain at 24 months follow up. Cervical and lumbar spine surgery for patients of this age group with stenosis carries very favourable results The improvement in both neurologic deficits, as well as quality of life issues compares favorably to similar surgeries in younger populations. The only exception was the poor prognosis post-operatively for the cervical myeolpathic patient already at Nurick Grade 5.

Several posters (AAOS poster 387 and 389) further examined osteoporotic vertebral fracture. Kanayama M (Hakodate, Japain) reported that the use of hook-rod instrumentation is recommended to treat radiculopathy associated with this fracture type. And although many patients with osteoporotic vertebral fracture are treated conservatively, there are patients who are not satisfied with the results of this treatment according to Arizono T (Fukuoka-city, Japan). Arizono found several predisposing factors to poor results in these patients and states that they may need a strict treatment such as body casting or vertebroplasty to achieve good pain relief.

INNOVATIVE APPLICATIONS

Orthopaedic research continues to progress rapidly as new methodology becomes available. DiGioia A (Pittsburgh, PA) and co-authors, members of the orthopaedic research society, conducted investigations on a wide range of topics relevant to improving our knowledge of the etiology, diagnosis, and treatment of musculoskeletal disease (AAOS Scientific Exhibit 87). Some of the current exciting areas of investigation include tissue engineering using stem cells, adipose-derived cells, and muscle cells; studies examining the effects of mechanical forces (stress and strain) on cells and tissues; newly-identified mediators of bone resorption (osteoprotegerin) and bone formation (PTH) that may play important roles in osteolysis and osteoporosis; gene expression in healing bone, ligament, and tendon; novel imaging techniques such as Fourier-transform infrared (FTIR) and double-quantum-filtered NMR; new biomaterials that may be used for implants and tissue regeneration, and gene therapy methods to induce expression of key molecules in healing bone, cartilage, ligament, tendon, meniscus, nerve and muscle.

Aseptic failure of fixation in total hip arthroplasty (THA) is known to be linked to early implant migration. Friedl G (Graz, Austria) and co-authors wanted to improve the early implant fixation by a single application of Zolendronic acid (ZOL) in patients with osteonecrosis of the femoral head, which previously was shown to be associated with higher stem migration and implant failure rate (AAOS, Poster 098). Forty-nine patients (mean age 61.0 ± 12.8) were randomly assigned to receive either i.v. ZOL 4mg (n=25) or saline solution (CTR, n=24) following THA. A total of 576 standardized radiographs were analyzed by the EBRA-Digital method at 0, 7, 26 weeks, and annually after surgery. Additionally, the Harris hip score was assessed during the median subsidence of 1.1 mm (range 0.3 to 2.3) and 1.3 mm (range 0.3 to 2.9) at 2 and 3 years respectively, and a migration of 0.8 mm (range 0.1 to 2.4) in ZOL (NS). EBRA-cup analysis demonstrated an increasingly transverse and vertical implant migration (i.e., medialization and cranialization of 0.6 mm each at 2 years) in CTR, while no cup-migration was detected in patients treated with ZOL 4 mg is sufficient to increase early impant fixiaton, which might improved the long-term durability of THA.