Subsidence of SB Charité total disc replacement and the role of undersizing.

PURPOSE: A possible complication after total disc replacement (TDR) is subsidence, presumably caused by asymmetric implantation, implant undersizing or reduced bone quality. This study aims to quantify the degree of subsidence of an SB Charité TDR, and investigate whether undersizing is related to subsidence. METHODS: A custom developed software package (Mathworks) reconstructed 3D bone-implant geometry. A threshold for subsidence was determined by comparing penetrated bone volume (PBV) and rotation angles. Inter- and intra-observer reproducibilities were calculated. Subsidence was correlated to undersizing. RESULTS: High inter- and intra-observer correlation coefficients were found for the method (R > 0.92). Subsidence was quantified as PBV 700 mm(3) combined with a rotation angle >7.5°. A reduced risk of subsidence was correlated to >60 and >62 % of the bony endplate covered by the TDR endplate for L4 and L5, respectively. CONCLUSIONS: A reproducible method to determine undersizing was developed. Thresholds were determined related to a reduced risk of subsidence.

Clinical outcomes of two revision strategies for failed total disc replacements.

PURPOSE: To compare mid-term clinical outcomes of two revision strategies for patients with failed SB Charité III total disc replacements (TDRs). METHODS: Eighteen patients with a failed TDR underwent posterolateral instrumented fusion (fusion group); in 21 patients, the TDR was removed and the intervertebral defect was filled with a bone strut graft, followed by an instrumented posterolateral fusion (removal group). Visual analogue scale (VAS) for pain and Oswestry Disability Index (ODI) were completed pre- and post-revision surgery. Intra- and post-operative complications of both revision strategies were assessed. RESULTS: Mean follow-up was 3.7 years (range 1.0-6.4) in the removal group and 4.4 years (range 0.7-11.0) in the fusion group. Although the removal group showed a significantly lower VAS and ODI score post-revision surgery as compared to preoperative (P < 0.01 and P = 0.01, respectively), no significant differences were found between the removal and fusion groups before and after revision surgery in VAS and ODI. A clinical relevant improvement in VAS and ODI was found in 47 and 21 % respectively in the removal group, and in 22 and 27 % respectively in the fusion group. Substantial complications were observed only in the removal group. CONCLUSIONS: Both procedures showed improvement clinically. There were no significant additional benefits of removing the TDR as compared to fusion alone at mid-term follow-up. The clinical decision to remove the TDR should be carefully weighed up against potential risks and complications of this procedure.

Clinical, radiological, histological and retrieval findings of Activ-L and Mobidisc total disc replacements: a study of two patients.

INTRODUCTION: This study evaluates the short-term clinical outcome, radiological, histological and device retrieval findings of two patients with second generation lumbar total disc replacement (TDR). MATERIALS AND METHODS: The first patient had a single level L4-L5 Activ-L TDR, the second patient a L4-L5 Mobidisc and L5-S1 Activ-L TDR. The TDRs were implanted elsewhere and had implantation times between 1.3 and 2.8 years. RESULTS: Plain radiographs and CT-scanning showed slight subsidence of the Activ-L TDR in both patients and facet joint degeneration. The patients underwent revision surgery because of recurrent back and leg pain. After removal of the TDR and posterolateral fusion, the pain improved. Histological examination revealed large ultrahigh molecular weight polyethylene (UHMWPE) particles and giant cells in the retrieved tissue surrounding the Mobidisc. The particles in the tissue samples of the Activ-L TDR were smaller and contained in macrophages. Retrieval analysis of the UHMWPE cores revealed evidence of minor adhesive and abrasive wear with signs of impingement in both TDR designs. CONCLUSION: Although wear was unrelated to the reason for revision, this study demonstrates the presence of UHMWPE particles and inflammatory cells in second generation TDR. Long-term follow-up after TDR is indicated for monitoring wear and implant status.

The Latest Lessons Learned from Retrieval Analyses of Ultra-High Molecular Weight Polyethylene, Metal-on-Metal, and Alternative Bearing Total Disc Replacements.

Knowledge regarding the in vivo performance and periposthetic tissue response of cervical and lumbar total disc replacements (TDRs) continues to expand. This review addresses the following four main questions: 1) What are the latest lessons learned from polyethylene in large joints and how are they relevant to current TDRs? 2) What are the latest lessons learned regarding adverse local tissue reactions from metal-on-metal, CoCr bearings in large joints and how are they relevant to current TDRs? 3) What advancements have been made in understanding the in vivo performance of alternative biomaterials, such as stainless steel and polycarbonate urethane, for TDRs in the past five years? 4) How has retrieval analysis of all these various artificial disc bearing technologies advanced the state of the art in preclinical testing of TDRs? The study of explanted artificial discs and their associated tissues can help inform bearing selection as well as the design of future generations of disc arthroplasty. Analyzing retrieved artificial discs is also essential for validating preclinical test methods.

Proximal tibiofibular synostosis as a possible cause of a pseudoradicular syndrome: a case report.

This paper presents a case report of persistent low back pain and suspected lumbar radiculopathy. A synostosis at the level of the proximal tibiofibular joint was diagnosed. After successful resection of the synostosis, the low back symptoms resolved completely. This is the first report of a proximal tibiofibular synostosis as a possible cause of referred pain proximally.

Removal of the Charité lumbar artificial disc prosthesis: surgical technique.

STUDY DESIGN: Descriptive study of a surgical technique for removal of the Charité total lumbar disc prosthesis. OBJECTIVE: To describe the surgical technique to remove total disc prosthesis and the occurrence of complications in disc removal surgery. SUMMARY OF BACKGROUND DATA: As lumbar total disc replacement seems to be more and more an accepted option to treat degenerative disc disease, the more will the need for removal increase in case of failure. METHODS: Twenty-nine Charité total disc prostheses were removed in 25 consecutive patients. To avoid mobilization of the adhered great vessels away from the spine, we address L4-L5 (or higher) anterolaterally from the left side, after mobilizing the psoas and exposing the lateral side of the disc level with the prosthesis. The approach to L5-S1 is similar to the implantation of the disc prosthesis. After removal of the prosthesis, the created defect is filled with a bone graft to accomplish an anterior fusion. This retrieval surgery is always combined with a posterior pedicle screw fusion. RESULTS: In our series, we had 4 intraoperative major vessel lesions without major blood loss; 1 of these cases developed a deep venous thrombosis of the left leg. In 1 case, we encountered a small colon lesion and in 1 patient, a lesion of the ureter resulting in resection of the kidney. In 1 case, we had profound bleeding from the bone beneath an S1 endplate, which resulted in a total blood loss of 5100 mL. In another patient, we planned to remove the disc prosthesis; however, because of rupture of the small intestine only posterior fusion was performed. CONCLUSIONS: Retrieval of a Charité artificial disc prosthesis is feasible, but it has its inherent risks.

Complications and reoperations of the SB Charité lumbar disc prosthesis: experience in 75 patients.

Artificial disc prosthesis show fair to good short- and mid-term results. Long-term results are becoming apparent now, however, the incidence of late complications with this procedure remain poorly understood. In this report we will analyse late complications and discuss our experiences with salvage operations in patients with persistent pain after SB Charité disc prosthesis implantation. Seventy-five patients with persistent leg and back pain after insertion of an artificial disc prosthesis were enrolled in the study. In this negative selection frequently occurring late-complications were subsidence, wear, adjacent disc degeneration, facet joint degeneration and migration. In 15 patients we performed a posterior fusion without disc removal, and in 22 patients we removed 26 prostheses and performed a posterior and anterior fusion. The visual analogue scale (VAS) and Oswestry were examined before the salvage operation and after a follow-up period of at least 1 year, which is not yet available in all patients. The VAS and Oswestry decreased in the posterior group (n = 10) respectively from 8.0 (SD 0.9) to 6.3 (SD 2.1) and from 57.0 (SD 17.0) to 44.6 (SD 20.4); and in the disc removal group (n = 14) respectively from 8.0 (SD 0.9) to 5.6 (SD 2.7) and from 56.3 (SD 14.0) to 43.0 (SD 20.7). Serious late complications may occur following total disc replacement. Removal of the SB Charité artificial disc is feasible but with inherent risks. Removal of the disc prosthesis gives slightly better results than posterior fusion alone after a follow-up of at least 1 year.

Polyethylene wear and rim fracture in total disc arthroplasty.

BACKGROUND CONTEXT: Polyethylene (PE) has been used in total disc replacements (TDRs) in Europe since the 1980s. However, the extent of surface damage of PE, including rim fracture and wear, after long-term implantation remains poorly understood. PURPOSE: The purpose of this study was to evaluate the magnitude and rate of PE wear and surface damage in TDRs. STUDY DESIGN: TDR components were retrieved from patients undergoing revision TDR surgery and conversion to fusion. PATIENT SAMPLE: Twenty-one implants (SB Charité III; DePuy Spine, Raynham, MA) were analyzed from 18 patients (12 female, 6 male) undergoing TDR revision surgery. The components were implanted between 1.8 and 16.0 years (average: 7.8 years) at L2-L3 (n=1), L3-L4 (n=1), L4-L5 (n=11), and L5-S1 (n=8). They were removed due to pain (in all cases) and were associated with subsidence (n=6), anterior migration (n=2), core dislocation (n=2), lateral subluxation (n=1), wear with wire marker fracture (n=1), end plate loosening (n=2), and osteolysis (n=1). OUTCOME MEASURES: Clinical information was collected from medical records and radiographs. Retrieval analysis included dimensional measurements and assessment of the extent and severity of PE surface damage mechanisms. METHODS: MicroCT scanning was used to identify the presence of internal cracks in the PE core and to scan the geometry of the retrievals. Light microscopy, coupled with white light interferometry, was used to evaluate the surface damage mechanisms at the dome and rim. RESULTS: The dominant wear mechanism was adhesive/abrasive wear at both the dome and rim. End plate penetration (dome wear) ranged from 0.1 to 0.9 mm (average: 0.3 mm), and was correlated with implantation time (Spearman's rho=0.48, p=.03). There was also evidence of macroscopic rim damage, including radial and transverse cracking, fracture, plastic deformation, and third-body damage. End plate penetration measured at the rims ranged from 0.02 to 0.8 mm (average: 0.3 mm). Cracks in the core were oriented transversely in 11 of 21 implants (52%), and radially around the rim in 11 of 21 implants (52%). Radiographic wire marker fracture, observed in 9 of 21 implants (43%), was always associated with deformation, cracking, or fracture of the PE rim. In two cases, a fractured wire marker became lodged in the articulating surface between the PE and the metallic end plate. CONCLUSIONS: This is the first study to quantitatively analyze the long-term PE damage mechanisms in contemporary TDRs. The TDRs displayed surface damage observed previously in both hip and knee replacements. Because of the evidence of increasing wear with implantation time, along with the demonstrated potential for osteolysis in the spine, regular long-term follow-up for patients undergoing TDRs is warranted.

Non-genetic expression of adolescent idiopathic scoliosis: a case report and review of the literature.

Treating children with idiopathic scoliosis can amaze someone at the many different ways in which the deformity can present. Most authors state that genetics stipulates the course of adolescent idiopathic scoliosis. This is mainly based on the high concordance in monozygotic twins. However, there is indication that environmental factors have influences on adolescent idiopathic scoliosis. This is the first report in which a monozygotic twin pair is described concordant for idiopathic scoliosis but with different apical levels, magnitudes and age at detection of scoliosis which stresses the importance of environmental factors.

Primary hyperparathyroidism and pathological fractures: a review.

Primary hyperparathyroidism is due most often to a parathyroid adenoma secreting parathyroid hormone. Elevated PTH levels cause bone resorption, the formation of polyostotic lesions and a reduction in bone mineral density, predisposing to pathological fractures. The final stage of this disease is osteitis fibrosa cystica. The authors review the literature about osteitis fibrosa cystica and the treatment options when a pathologic fracture occurs.

Infection after anterior spinal fusion for idiopathic scoliosis using the Cotrel-Dubousset-Hopf system: A clinical case series of three patients.

BACKGROUND: Three patients with late-onset infection after multilevel instrumented anterior spinal fusion for idiopathic scoliosis, using the Cotrel-Dubousset-Hopf (CDH) system, are presented. The CDH-system is an anterior instrumentation with high biomechanical stability and rigidity, ensuring a stable primary fixation. Unlike after posterior spinal fusion, infection after anterior spinal fusion (ASF) for idiopathic scoliosis has rarely been reported. METHODS: The files of three patients who developed an infection after ASF for scoliosis using the CDH-system, were reviewed. The clinical presentation and diagnostic and therapeutic options are discussed. RESULTS: All three patients had a late-onset infection of the CDH-system, which was difficult to diagnose because of nonspecific symptoms. Radiographs and technetium bone scan appeared to be of low value. When an abscess was present, this could accurately be diagnosed with MRI or CT imaging. Operative treatment with implant removal and antibiotic therapy was successful in all cases. CONCLUSION: Late onset infections after ASF using the CDH-system presented with few and nonspecific symptoms. The clinical presentation was mainly characterized by vague abdominal- or back-pain after an interval of normal postoperative recovery, moderately raised infection parameters and inconclusive findings with imaging modalities. As treatment, implant removal, debridement and parenteral antibiotics are recommended. It should be noted though that implant removal poses serious risks for vascular and visceral structures.

Intrinsically radiopaque hydrogels for nucleus pulposus replacement.

Degeneration of the intervertebral disc is the most common cause of back pain. In case of early stage degenerative disc disease or traumatic herniations, a suitable treatment may be to replace the nucleus pulposus, preserving the annulus fibrosus. Eight new hydrogel biomaterials were prepared and studied for their potential as a nucleus replacement. The hydrogels were designed according to the following criteria: (i), they should exhibit adequate radiopacity; (ii), they should be non-cytotoxic; (iii), implantation in the dry state and subsequent swelling in situ to fill the entire nucleus cavity; (iv), after swelling they should match the physical-mechanical properties of the native nucleus. The approach was to use copolymers consisting of 2-(4'-iodobenzoyl)-oxo-ethyl methacrylate (4IEMA) and a hydrophilic building block (either N-vinyl-2-pyrrolidinone (NVP) or 2-hydroxyethyl methacrylate (HEMA)); 4 copolymers of NVP/4IEMA and 4 copolymers of HEMA/4IEMA in different compositions (5, 10, 15 and 20 mol% 4IEMA). The study comprised 1H-NMR analysis of the copolymerization reaction NVP+4IEMA. Furthermore, the copolymers were studied with respect to their swelling behavior, mechanical properties, cytotoxicity in vitro and X-ray contrast. Hydrogels with 5 mol% 4IEMA appear to meet all criteria: they are non-cytotoxic, have adequate physical-mechanical properties and feature sufficient radiopacity in a realistic model. The potential implications of these new results with respect to treatment of degenerative disc disease are discussed briefly.

Predictability of spontaneous thoracic curve correction after anterior thoracolumbar correction and fusion in adolescent idiopathic scoliosis. A retrospective study on a consecutive series of 29 patients with a minimum follow-up of 2 years.

BACKGROUND CONTEXT: According to the Lenke classification, a Type 5 adolescent idiopathic scoliosis can be surgically treated with selective anterior thoracolumbar or lumbar fusion. PURPOSE: This study aims to predict the spontaneous correction of the unfused thoracic curve after anterior thoracolumbar fusion and to study whether age is of influence on this predictability. STUDY DESIGN: Retrospective study on a consecutive series of patients. PATIENT SAMPLE: Of a consecutive series of 38 patients with idiopathic thoracolumbar scoliosis, Lenke type 5, 29 patients were included in the study. All patients were treated with anterior spinal fusion and instrumentation. A minimum follow-up of 2 years was available for all patients (mean, 4 years; range, 2-17 years). To investigate the influence of age on the outcome, we divided the group into two subgroups: an adolescent (n=13) and an adult age group (n=16). The mean age in the adolescent group was 17 (range, 13-21 years) and 38 years (range, 22-54 years) in the adult group. OUTCOME MEASURES: Physiological measures include coronal Cobb angle, apical vertebral translation (AVT) and apical vertebral rotation (AVR), shoulder tilt, trunk shift, L4 tilt, and pelvic tilt. Relative corrections were computed for the thoracolumbar and thoracic curves in each patient using the following formula: (preoperative curve-postoperative curve)/preoperative curve×100 (%). The correlation coefficient between the relative (%) corrections of the thoracic and thoracolumbar curves was calculated for the whole group as for the two age subgroups. METHODS: For radiographic evaluation, we used standing anteroposterior and lateral projections of the thoracolumbar spine to determine Cobb angle, AVT and AVR, and coronal balance. RESULTS: Both the thoracolumbar and thoracic curves in the whole group improved after surgery (45% and 19%, respectively, p<.01). In the adolescent age group, a significant correlation between the relative (%) correction of the thoracolumbar curve and the relative (%) correction of the thoracic curve was found (R=0.704; p=.01) and between age and relative (%) correction of the thoracic curve (R=-0.805; p<.01). CONCLUSIONS: These results show that the spontaneous correction of the thoracic curve is a reflection of the thoracolumbar curve correction in adolescent thoracolumbar idiopathic scoliosis. Moreover, the predictability of the thoracic curve correction in the individual patient seems to decrease with increasing age of the patient.

Are periprosthetic tissue reactions observed after revision of total disc replacement comparable to the reactions observed after total hip or knee revision surgery?

STUDY DESIGN: Comparative study. OBJECTIVE: To compare periprosthetic tissue reactions observed after total disc replacement (TDR), total hip arthroplasty (THA), and total knee arthroplasty (TKA) revision surgery. SUMMARY OF BACKGROUND DATA: Prosthetic wear debris leading to particle disease, followed by osteolysis, is often observed after THA and TKA. Although the presence of polyethylene (PE) particles and periprosthetic inflammation after TDR has been proven recently, osteolysis is rarely observed. The clinical relevance of PE wear debris in the spine remains poorly understood. METHODS: The number, size, and shape of PE particles, as well as quantity and type of inflammatory cells in periprosthetic tissue retrieved during CHARITÉ TDR (n = 22), THA (n = 10), and TKA (n = 4) revision surgery were compared. Tissue samples were stained with hematoxylin/eosin and examined by using light microscopy with bright field and polarized light. RESULTS: After THA, large numbers of PE particles of size less than 6 µm were observed, which were mainly phagocytosed by macrophages. The TKA group had a broad size range with many larger PE particles and more giant cells. In TDR, the size range was similar to that observed in TKA. However, the smallest particles were the most prevalent with 75% of the particles being less than 6 µm, as seen in revision THA. In TDR, both macrophages and giant cells were present with a higher number of macrophages. CONCLUSION: Both small and large PE particles are present after TDR revision surgery compatible with both THA and TKA wear patterns. The similarities between periprosthetic tissue reactions in the different groups may give more insight into the clinical relevance of PE particles and inflammatory cells in the lumbar spine. The current findings may help to improve TDR design as applied from technologies previously developed in THA and TKA with the goal of a longer survival of TDR.

Repair of gluteus medius muscle avulsion following transgluteal hip replacement.

During a transgluteal approach to the hip joint the anterior part of the gluteus medius and minimus muscles are detached and subsequently reattached to the greater trochanter. Avulsion or rupture of these muscles may result in weak abduction, pain and/or instability. 15 patients with these symptoms were treated, of whom 13 had muscle avulsion at surgery. Reinsertion of these muscles resulted in improvement of pain in 6 (55%) patients and improvement of walking ability in 9 (82%) patients. Reinsertion of avulsed gluteal muscles after transgluteal approaches to the hip may relieve pain and improve walking ability.

Submicron sized ultra-high molecular weight polyethylene wear particle analysis from revised SB Charité III total disc replacements.

Submicron sized particles are frequently observed in retrieved total hip and knee periprosthetic tissues and appear to be critical in the activation of the phagocytic inflammatory response. In this paper the concentration, size and shape of ultra-high molecular weight polyethylene (UHMWPE) wear particles between 0.05 and 2.00µm were determined after isolation from periprosthetic tissues from retrieved lumbar SB Charité III total disc replacements (TDR) using scanning electron microscopy (SEM). For comparison, UHMWPE wear particles were isolated from γ-radiation-air sterilized total hip arthroplasty (THA) revision tissues. The mean concentration of UHMWPE particles in TDR tissues was 1.6×10(9)g(-1)tissue (range 1.3-2.0), which was significantly lower than the concentration of 2.3×10(9)g(-1) THA revision tissue (range 1.8-3.2) (P=0.03). The mean particle size (equivalent circular diameter: TDR, 0.46µm; THA 0.53µm, P=0.60) and mean shape were comparable between TDR and THA (aspect ratio: TDR, 1.89; THA, 1.99, P=0.35; roundness: TDR, 0.58; THA, 0.56, P=0.35). However, the TDR particles tended to be smaller and more round. Although no correlations were found between visible damage to the UHMWPE core and the concentration or shape of the UHMWPE particles, a positive correlation was found between increasing particle size and increasing rim penetration of the TDR core (P=0.04). The presence of UHMWPE particles of similar size and shape in TDR tissue, albeit lower in concentration, might explain why, unlike THA, pain rather than osteolysis is the major reason for revision surgery.

In vivo deformation, surface damage, and biostability of retrieved Dynesys systems.

STUDY DESIGN: Retrospective retrieval analysis. OBJECTIVE: To evaluate wear, deformation and biodegradation within retrieved polycarbonate urethane (PCU) components of Dynesys systems. SUMMARY OF BACKGROUND DATA: The Dynesys Dynamic Stabilization System (Zimmer Spine) consists of pedicle screws (Ti alloy), polycarbonate urethane (PCU) spacers, and a polyethylene-terephthalate cord. METHODS: Seventeen retrieved (mean implantation: 2.5 years, range: 0.7-7.0 years) and 2 exemplar implant systems were available. Reasons for revision were persistent pain (16/17), infection (1/17), and/or screw loosening (11/17), with 1/17 case of implant migration. Optical microscopy, microCT, and scanning electron microscopy were conducted to evaluate PCU spacer wear and deformation. Attenuated total reflectance Fourier transform infrared spectroscopy was used to assess spacer surface chemical composition. RESULTS: Retrieved spacer components exhibited permanent bending deformation (mean: 4.3°, range: 0.0°-15.8°). We observed evidence of PCU spacer contact with pedicle screws, cords, and surrounding bony structures (74/75, 69/75, and 51/75 spacers, respectively). Relatively infrequent damage modes included PCU fracture (1/75 spacers) or cracking (2/75 spacers), as well as pedicle screw fracture (3/103 screws). PCU degradation products were identified in 10/75 spacers, which represented retrievals having significantly longer implantation times (mean: 4.3 years, range: 1.0-7.0 years). Of these spacers, 8/10 had degradation peaks identified along the side of the spacer where the material would have been in contact with bodily fluid. CONCLUSION: PCU spacers from retrieved Dynesys systems exhibited permanent deformation, focal regions of in vivo wear and surface damage. Chemical changes associated with PCU biodegradation were associated with longer-term retrievals. The most frequently observed complication was pedicle screw loosening, with 3 incidences of screw breakage in 2 patients. These retrieval data provide a crucial basis for developing in vitro tests to simulate in vivo damage and degradation of posterior dynamic motion preservation implants. Longer-term retrievals, as well as retrievals that include more recent design features (e.g., HA coating), will be useful to provide a greater context for the clinical implications of our short-term observations.

The natural history of polyethylene oxidation in total disc replacement.

STUDY DESIGN: This study is an evaluation of wear and oxidation in retrieved total disc replacements (TDRs). Forty-eight CHARITE TDRs were retrieved from 41 patients after 7.8 years of average implantation. All implants were removed because of intractable back pain and/or facet degeneration. Three unimplanted implants served as controls. OBJECTIVE: Our aim was to determine whether gamma-sterilized polyethylene components implanted in the spine oxidize in vivo, and if so, whether polyethylene oxidation has clinical relevance for the long-term performance of TDRs. SUMMARY OF BACKGROUND DATA: The natural history of polyethylene oxidation following gamma sterilization and long-term implantation in the spine has not yet been investigated. METHODS: Oxidation and oxidation potential were measured at the rim and dome of 47 components using Fourier transform infrared spectroscopy. The wear patterns of each retrieved polyethylene core were analyzed at the rim and dome. RESULTS: Oxidation was significantly higher at the rim, as compared with the dome of the cores. Hydroperoxide index was also significantly higher at the rim, as compared with the dome. Dome penetration rate was negatively correlated to implantation time (P < 0.0001) but not correlated to oxidation or hydroperoxide index (P > 0.05). Implants with evidence of chronic rim loading had higher rim oxidation. CONCLUSION: The data support our hypothesis that, for the historical packaging methods employed by the manufacturer, polyethylene oxidation and oxidation potential were significantly higher at the rim as opposed to the dome. The mechanism is governed by access to oxygen in vivo and may be accelerated under certain combined modes of repeated rim loading. Our findings have clinical significance in cases of chronic impingement, when the rim has to support repeated loading for the lifetime of the implant.

Retrieval analysis of motion preserving spinal devices and periprosthetic tissues.

This article reviews certain practical aspects of retrieval analysis for motion preserving spinal implants and periprosthetic tissues as an essential component of the overall revision strategy for these implants. At our institution, we established an international repository for motion-preserving spine implants in 2004. Our repository is currently open to all spine surgeons, and is intended to be inclusive of all cervical and lumbar implant designs such as artificial discs and posterior dynamic stabilization devices. Although a wide range of alternative materials is being investigated for nonfusion spine implants, many of the examples in this review are drawn from our existing repository of metal-on-polyethylene, metal-on-metal lumbar total disc replacements (TDRs), and polyurethane-based dynamic motion preservation devices. These devices are already approved or nearing approval for use in the United States, and hence are the most clinically relevant at the present time. This article summarizes the current literature on the retrieval analysis of these implants and concludes with recommendations for the development of new test methods that are based on the current state of knowledge of in vivo wear and damage mechanisms. Furthermore, the relevance and need to evaluate the surrounding tissue to obtain a complete understanding of the biological reaction to implant component corrosion and wear is reviewed.