Clinical and Radiologic Outcomes of a Fully

The Journal of Arthroplasty xxx (2015) xxx–xxx

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Clinical and Radiologic Outcomes of a Fully Hydroxyapatite-Coated Femoral Revision Stem: Excessive Stress Shielding Incidence and its Consequences Ilknur Sanli, MD, Jacobus Johannes Christiaan Arts, PhD, Jan Geurts, MD Department of Orthopaedic Surgery, Research School Caphri, Maastricht University Medical Centre, Maastricht, The Netherlands

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Article history: Received 17 March 2015 Accepted 11 August 2015 Available online xxxx Keywords: femoral stem revision hydroxyapatite stress-shielding

a b s t r a c t Stress shielding remains a concern in total hip arthroplasty. The consequences of stress shielding in hydroxyapatite-coated femoral component revisions were evaluated in a prospective cohort study. A total of 106 patients operated on by revision total hip arthroplasty were identified. Sixty-three patients were eligible for clinical and radiologic assessment of osseointegration, bone remodeling, and stress shielding. Five patients showed evidence of excessive stress shielding. One patient experienced a periprosthetic fracture. No adverse events occurred in the remaining patients with a low rate of thigh pain and reliable osseointegration. This is the only available study concerning mid- to long-term consequences of excessive stress shielding in hydroxyapatite-coated revision stems. We advocate surgeons using these stems to remain vigilant and be aware of possible stress shielding side effects. © 2015 Elsevier Inc. All rights reserved.

Failure of total hip arthroplasty (THA) continues to present a significant clinical challenge. The number of revision hip arthroplasties performed each year has increased exponentially over the last half century, and these increases have been sustained over the first 5 years of the new millennium: numbers are between 4% and 26% worldwide for revision hip surgery [1]. Kurtz et al [2] constructed a model to predict the future rate of revision THA in the United States from 2005 to 2030. They projected a 137% increase in 2030. As the revision burden increases, achieving reliable and durable fixation between the implant and host bone presents a challenge. Femoral component loosening is a common mechanical failure and is of great concern in the orthopedic literature. Stress shielding is a mechanical cause of bone loss [3]. This phenomenon is caused because the natural stress distribution through the femur is altered. The implant will carry a portion of the load and distribute some of the load to the midshaft region of the femur. This causes a reduction of stress in some

One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements refer to http://dx.doi.org/10.1016/j.arth.2015.08.037. Conflict of interest statement: Each author certifies that he or she has no commercial associations (consultancies, stock ownership, equity interest, patent/licensing arrangements, etc) that might pose a conflict of interest in connection with the submitted article. Reprint requests: Jan Geurts, MD, Department of Orthopaedic Surgery, Research School Caphri, Maastricht University Medical Centre, P. Debyelaan 25, PO Box 5800, 6202 AZ Maastricht, The Netherlands.

areas of the remaining bone, primarily in the proximal metaphyseal region. If bone experiences little or no stress, there will be a loss of bone mass in that region. This is known as bone resorption and can cause the prosthesis to loosen from the bone. Potential adverse effects of stress shielding may be stem, bone, or interface failure or deficient bone stock when a (re)-revision is required. Clinical and animal experimental studies have revealed factors influencing stress shielding, which include stem stiffness, stem shape, stem coating extent, fit between stem and bone, bone quality, and patient weight [3,4]. Cemented and proximally porous-coated femoral revision stems have demonstrated disappointing clinical results to date [5-11]. An alternative in femoral revisions to bypass the problem of stress shielding is the use of fully hydroxyapatite (HA)–coated femoral stems. Such stems show favorable results with mechanical failure rates of 1% to 6.9% as compared to higher failure rates using cement revision arthroplasty with similar length of clinical follow-up (FU) [12-17]. Hydroxyapatite coating has been shown to promote osteoconduction in both primary and revision cases [18,19]. As a result of these biologic properties, it is feasible that the requirement for augmentary bone could be reduced or eliminated in many if not most cases. Studies using radiostereometric analysis showed reduced migration of HA-coated prosthetic components and better radiographic results and survival rates with HA-coated stems when compared with identical press-fit components [20]. Although the preliminary results of fully HA-coated femoral stems show promising clinical results, concerns over stress shielding still exist. There is paucity of evidence for the midto long-term clinical performance of fully HA-coated femoral revision implants. We present our institutional results for the mid- to long-term outcome of the Restoration HA femoral revision stem (Stryker, Mahwah,

http://dx.doi.org/10.1016/j.arth.2015.08.037 0883-5403/© 2015 Elsevier Inc. All rights reserved.

Please cite this article as: Sanli I, et al, Clinical and Radiologic Outcomes of a Fully Hydroxyapatite-Coated Femoral Revision Stem: Excessive Stress Shielding Incidence and its Consequences, J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.08.037

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I. Sanli et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx

NJ), in which we discuss the clinical performance, radiologic outcome, and clinical consequences of stress shielding. Methods Patient Selection A cohort analysis was performed in a prospective study design of all patients who underwent revision THA at our institution using a fully HA-coated revision hip stem between January 1998 and January 2007. In the study period, 106 patients received a fully HA-coated Restoration stem. All patients were included with a minimum FU of 60 months. There were no exclusion criteria. A total of 63 patients were eligible for final assessment. Primary study outcome was the clinical performance and radiologic outcome of the aforementioned revision stem with specific focus on stress shielding occurrence. The surface of this femoral implant is acid-etched titanium alloy with a circumferentially applied 50-μm HA surface coating. The distal portion of the stem is 0.5 mm tapered, and the shaft-neck angle is anatomical 127°. The stem comes in multiple diameters up to 22 mm and lengths of 155, 205, and 265 mm (Fig. 1). All procedures were performed by or under the supervision of 2 specialist hip surgeons. In all patients, a posterior approach to the hip was used, with an extended trochanteric osteotomy if required to facilitate cement removal. In most of the patients, a 1-stage revision was achieved. In the postoperative treatment protocol, 3 patients were treated with non–weight bearing; 51 patients, with partial weight bearing; 6 patients had full weight bearing; and in 3 patients, the postoperative weight-bearing protocol was not specified. This study was approved by the medical ethical committee of our institution (no. 08-4-048) and conducted according to current Good Clinical Practice and ISO 14155 guidelines. All patients signed informed consent before participation in this study.

Clinical Assessment A clinical examination was performed postoperatively at 6 weeks, 6 months, 12 months, and after minimum 60 months clinical FU, and the following were recorded: range of motion, incidence of thigh pain, and the level of physical activity. Furthermore, the Oxford Hip Score (OHS), Harris Hip Score (HHS), and the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) scores were assessed. Radiologic assessment of osseointegration, bone remodeling, and assessment of stress shielding in all Gruen zones were matched to clinical assessment time points. Radiologic outcome was assessed postoperatively at 6 weeks; at 6, 12, and 60 months; and at the last FU visit. A zonal analysis of the radiolucent lines, as outlined by Engh et al [13], was used to catalog the relevant changes in bone morphology and the bone implant interface characteristics in all Gruen zones. Radiologic outcome was assessed by 2 reviewers.

Results Patients Forty-three patients were lost to FU: 3 patients moved abroad, 1 patient died, 1 patient had significant Parkinson, and 38 patients withdrew consent because of refusal to participate in the study or incomplete adherence to FU schedules. A total of 63 patients were eligible for final assessment. The patients (38 females and 25 males) had an average age at the time of surgery of 58 years (range, 27-76 years), body mass index of 27 kg/m2 (range, 17-41 kg/m 2), and mean American Society of Anesthesiologists classification of 2 (range, 1-3). The hip pathologies that necessitated primary arthroplasty procedures were arthritic conditions, avascular necrosis, trauma, and congenital dysplasia. In 4 patients, the revision stem was used in a primary arthroplasty procedure. The reasons for the revision arthroplasty procedures were aseptic loosening of 1 or more components (22 patients), recurrent dislocations (8 patients), pain (6 patients), polyethylene wear (5 patients), malposition (4 patients), fracture (6 patients), girdlestone situation (2), and infection (1 patient). The mean revision rate was 3 revisions in the whole study population (Table).

Table Baseline Demographic Data of Patient Cohort.

Fig. 1. Restoration HA stem. Stem fabricated of titanium alloy roughened by a chemical etching process. Hydroxyapatite is plasma sprayed over the entire length of the stem. Designed with a large proximal cross-section to provide for improved load distribution over a broad area. Distally, the design incorporates a cylindrical section to more effectively use the available bone of the diaphysis. The stem design incorporates a physiologic 127° neck-stem angle, neck length ranges, and a C-taper head to provide the surgeon with the ability to restore near-anatomical head position for proper leg length and biomechanical function.

Baseline Characteristics

n = 63

Age (y), mean (SD) Male Female BMI, n (%) b30 N30 N/A ASA classification n (%) I II III IV N/A Reason revision n (%) Pain Septic loosening Aseptic loosening Recurrent dislocations Polyethylene wear Fracture Girdlestone situation Malposition N/A

58 (27-76) 25 (40) 38 (60) 44 (70) 14 (22) 5 (8) 12 (19) 35 (56) 5 (8) – 11 (17) 6 (10) 1 (2) 22 (35) 8 (13) 5 (8) 6 (10) 2 (3) 4 (6) 9 (14)

Abbreviations: BMI, body mass index; N/A, not available; ASA, American Society of Anesthesiologists.



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Complications There were 12 intraoperative fractures; 8 fractures of the shaft were stabilized by cerclage wires; and in 1 patient, plate fixation was used during surgery. All other fractures were treated conservatively, and regardless of treatment, all intraoperative fractures healed. Autologous bone graft was used in 5 femoral and 14 acetabular reconstructions to supplement osseointegration. A cup revision was performed in 35 patients. In the FU period, deep infections were seen in 2 patients and were successfully treated with debridement. Dislocations were only seen in 1 hip needing acetabular component revision. Two patients developed a periprosthetic fracture (calcar region); 1 patient developed the fracture spontaneously. The other patient had radiologic evidence of excessive stress shielding experiencing the periprosthetic fracture after a low-impact fall. No adverse events occurred in the remaining patients with evidence of radiologic excessive stress shielding. Clinical Outcomes The mean range of motion was 195° (range, 110°-310°). Two patients attended heavy labor/sports, 31 patients attended light labor/occasional sports, and 30 patients were semisedentary. At the latest review, the mean HHS was 89 points (32-100) with 51 (81%) patients being graded as good or excellent (score of ≥ 80). The mean WOMAC score was 26 points (range, 1-88), and OHS was 26 points (range, 12-55). In the FU period, 38 patients were pain free, 17 patients had occasional pain, 5 patients had mild pain with occasional use of medication, 1 patient had moderate limited activities of daily living, and 2 patients had severe pain with limited activities of daily living. There were only 2 patients in the FU period with midthigh pain. Radiologic Outcome Review of the femoral components revealed that all stems were stable with evidence of osseointegration. The mean Engh and Massin score was 18 (range, 1-27). Five patients (7.94%) in our cohort study had radiologic evidence of excessive stress shielding that developed with 5year postoperative time window. At 60-month FU, only 1 patient had an adverse event. In this patient, aseptic loosening was treated by revision of the cemented straight stem Muller prosthesis to the fully coated HA femoral stem and a cemented cup type SHP (Biomet Orthopaedics, Warsaw, IN) (Fig. 2A and B). Two years later, extensive bone remodeling around the femoral stem with cortical thickening around the distal stem was observed (Fig. 3), and on the 4-year postoperative radiographs, progressive loosening of Gruen zones 2, 3, 5, 6, 7, 9, 10, 12, and 13 was observed. A Vancouver type B3 periprosthetic femoral fracture was the consequence after a low-impact fall. The fracture was treated with a femoral strut graft, plate fixation, and cerclage wires (Fig. 4). Patient was followed up in a regular FU protocol with partial weight bearing during the first 3 months postoperatively and full weight bearing thereafter. Postoperatively, a leg length difference of 3.5 cm was observed, which was treated with a shoe modification. The remaining postoperative course was uncomplicated with a consolidated fracture and an HHS of 90 points at the last FU at 5 years. The remaining 4 patients with excessive stress shielding remained asymptomatic so far, but they are followed up more rigorously (Fig. 5A-C). Discussion Unfortunately, stress shielding can be problematic both in primary and revision surgery. Clinical implications of excessive stress shielding are, nevertheless, unknown. Does excessive stress shielding with a vanishing femur alter outcome? What are the implications for FU and treatment? We report the clinical consequences of excessive stress shielding in a fully HA-coated femoral revision stem. In our study, 7.94% of our patients showed evidence of excessive stress shielding. At

Fig. 2. (A) Anteroposterior radiograph with evidence of loosening of the left Muller THA. (B) Postoperative anteroposterior radiograph: dense corticalis in all Gruen zones after revision of the left hip with a fully coated HA femoral stem (Restoration HA; Stryker) and a cemented cup-type SHP.

the final FU, only 1 of these patients experienced a periprosthetic fracture after a low-impact fall. No adverse events occurred in the remaining patients with evidence of radiologic excessive stress shielding. Review of the femoral components revealed that all stems were stable with evidence of osseointegration. There were no re-revisions of the femoral component for aseptic loosening. Femoral component loosening is a common reason for mechanical failure of a THA, and it remains a great concern in the orthopedic literature. Stress shielding is a mechanical cause of bone loss and is characterized by adaptive remodeling changes in the proximal femoral cortex after stem implantation. The proposed mechanism of stress shielding is based on Wolff's law of remodeling [3]. The redistribution of stress results in a decrease of the bone mineral density around the proximal femur, which may influence the longevity of the prosthesis. The location where stress shielding occurs can be determined in a finite element model [21]. The analysis of Swanson and Freeman [21] compared the stress distribution that occurred in intact (without implant) and after implantation at 16 different points along medial and lateral sides. The stress in each point was reduced after the implant had been inserted into the femur. This reduction occurred both on the medial and lateral sides. The biggest change in stress distribution occurred at the proximal medial part. Potential adverse effects of stress shielding may be stem, bone, or interface failure, due to reduced bone stock in combination with impact loading, or insufficient bone stock being available when a (re-)revision is required. Clinical and animal experimental studies have revealed factors governing stress shielding, which include stem stiffness and coating extent, stem shape, fit, bone quality, and patient weight [4]. The goals of revision femoral arthroplasty are to achieve rotational stability of the implant, to prevent axial migration, and to


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Fig. 3. Ongoing bone remodeling in the following years postoperatively. Progressive bone loss in Gruen zones 2, 3, 5, 6, 7, 9, 10, 12, and 13. (A) Two years postoperatively. (B) Four years postoperatively.

reproduce normal hip biomechanics. The loss of bone and sclerotic endosteal surface in the proximal femur contribute to the difficulty in achieving mechanical stability. If bone stock is poor or if endosteal cancellous bone is lacking, cemented implants are not well suited to

achieve these goals and other surgical methods give better results. The quality of periprosthetic bone determines the risk of periprosthetic fracture and also defines the complexity of revision surgery if needed. Better periprosthetic bone preservation can decrease the need for complex

Fig. 4. A Vancouver type B3 periprosthetic fracture after a low-impact fall 4 years later, treated with a femoral strut graft, plate fixation, and cerclage wires.



Fig. 5. (A) Anteroposterior x-ray of the left hip 6 months postoperatively showing osteolysis in Gruen zones 2 and 3. (B and C) Anteroposterior x-ray 2 years and 6 years postoperatively with evidence of ongoing stress shielding in Gruen zone 2 and cortical hypertrophy in Gruen zones 3 and 5.

reconstruction. Chandran et al [22] showed that bone density is better preserved around the uncemented stem compared with the cemented stem in the long term at 12 years of FU. The aim of using these uncemented components is to achieve biological fixation by ingrowth of endosteal bone with new bone formation within the porous surface structure of the implant. Most uncemented revision stems are designed to bypass the proximally damaged zone and to achieve initial stability from press-fit distally [15,16]. Distally anchored stems have shown good clinical results in the short to medium terms but have the disadvantage of potentially inducing severe stress shielding, thus causing further bone loss proximally [23]. For this purpose, extensively porous-coated stems have been used with good results. Extensively porous-coated stems get fixated both

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proximally and distally. Severe stress shielding with fully porouscoated stems ranges from 6% to 19.5%; mechanical loosening was reported up to 11% [14,17]. Hydroxyapatite has been used to enhance fixation of cementless stems by encouraging bone ingrowth. Results of HA-coated femoral stems in the primary THA have been excellent. A recent meta-analysis showed that HA coating compared to porous coating could improve postoperative HHS, with a reduction of incidence of thigh pain and femoral osteolysis. In addition, the subgroup analyses found that the longer the duration of FU, the better the advantage of HA coating over porous coating for the HHS and survivorship from aseptic loosening [24]. However, little has been published on HA-coated stem use in the revision setting [15,16]. The fully HA-coated revision stem has the potential for improved fixation with less fibrous ingrowth, less thigh pain, and less stress shielding. In the literature, there are only 2 studies reporting on the early clinical and radiographic results with use of this stem [15,16]. In the series of Lawrence et al [15] of the 22 femoral revisions in 20 patients, there was a 0% incidence of mechanical loosening at average FU of 3.2 years (2-6.3 years). All 22 femoral components had evidence of bone ingrowth. The extensively coated HA stem in this series produced excellent clinical results with a low incidence of thigh pain (4.5%) and severe stress shielding (4.5%). Crawford et al [16] showed comparable clinical results with 59 reviewed femoral component revisions with extensively coated stems, which showed no cases of severe stress shielding and only 1 case of loosening in an FU period of 3.3 years (range, 2-5 years). Periprosthetic bone remodeling secondary tot stress shielding may contribute to increased pain or decreased function, fracture of the femur or the femoral component, loss of fixation of implant, increased prevalence or severity of osteolysis, and difficulty in performing a revision. The prevalence of these adverse events is limitedly reported. Bugbee et al [25] retrospectively studied 215 patients with an uncemented primary hip arthroplasty by use of an anatomical medullary locking hip system. Forty-four patients (45 hips) who had evidence of pronounced femoral bone remodeling on the 2-year radiographs were clinically followed up with a minimum of 10-year FU. In this study population, 2 patients had a reoperation, but neither procedure involved the femoral component; 1 patient had a revision of a loose acetabular component and 1 had an exchange of a polyethylene liner. Forty-one patients (93%) reflected decreased pain and improved function as well as a high rate of overall satisfaction in the questionnaires. This is the only available study concerning the mid- and long-term clinical consequences of excessive stress shielding. The rate of adverse clinical events (revision, reoperation, and osteolysis) in the present series of patients who had bone remodeling of the proximal part of the femur was lower than in studies of patients who had the same implant or a similar implant and who were followed up for a similar period. Limitations of this study are the limited number of patients. Another limitation of the study is that we used 1 period for measurement of HHS, WOMAC, and OHS outcome, at the end of the FU period. Another shortcoming of the study is that we assessed stress shielding on plain radiographs and did not use a Dual-energy X-ray Absorptiometry or computed tomography–assisted osteodensitometry. Dual-energy X-ray Absorptiometry scanning has a high reproducibility and has the ability to detect and quantify even small osteolytic lesion with high precision. Computed tomography–assisted osteodensitometry accurately differentiates cortical and cancellous bone density changes around the femoral component after THA. The study of Muir et al [23] showed us that the Engh Grading Scale is a reliable tool especially when used by a single, experienced arthroplasty surgeon. In our study, a series of radiographs were independently reviewed by an orthopedic surgeon and orthopedic resident. We believe that the Engh scale has a prominent role in research for establishing the long-term success of THA. The study of Bugbee et al could suggest that no clinical problems are to expect because of stress shielding [25]. Hence, these are retrospective data with a small series of patients. There is no literature about the implications of excessive stress shielding for FU and treatment. Should we


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follow these patients more strictly? When do we decide to perform a reintervention? In our institution, we follow up patients with this kind of problem on a 6-monthly basis. When patients become symptomatic with intolerable pain, gait difficulties, or other disabling symptoms, reintervention should be considered. Although the preliminary results of fully HA-coated revision stems are satisfactory, progressive bone loss through stress shielding remains a problem. Although such problems have not been manifested as severe, the preservation of femoral bone stock is an important and desirable goal. We advocate surgeons using fully HA-coated femoral stems to remain vigilant and be aware of possible stress shielding side effects.

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