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1.  Wear of a 5 Megarad Cross-linked Polyethylene Liner: A 6-year RSA Study 
One cross-linked polyethylene (XLPE) liner is manufactured using a lower dose of radiation, 5 Mrad, which may result in less cross-linking. The reported in vivo wear rate of this XLPE liner in patients undergoing THA has varied, and has included some patients in each reported cohort who had greater than 0.1 mm/year of wear, which is an historical threshold for osteolysis. Previous studies have measured wear on plain radiographs, an approach that has limited sensitivity.
We therefore measured the amount and direction of wear at 6 years using Radiostereometric analysis (RSA) in patients who had THAs that included a cross-linked polyethylene liner manufactured using 5 Mrad radiation.
We prospectively reviewed wear in 30 patients who underwent primary THAs with the same design of cross-linked acetabular liner and a 28-mm articulation. Tantalum markers were inserted during surgery and all patients had RSA radiographic examinations at 1 week, 6 months, 1, 2, and 6 years postoperatively.
The mean proximal, two-dimensional (2-D) and three-dimensional (3-D) wear rates calculated between 1 year and 6 years were 0.014, 0.014, and 0.018 mm/per year, respectively. The direction of the head penetration recorded between 1 week and 6 years was in a proximal direction for all patients, proximolateral for 16 of 24 patients, and proximomedial for eight of 24 patients.
The proximal, 2-D and 3-D wear of a XLPE liner produced using 5 Mrad of radiation was low but measurable by RSA after 6 years. No patients had proximal 2-D or 3-D wear rates exceeding 0.1 mm/year. Further followup is needed to evaluate the effect of XLPE wear particles on the development of long-term osteolysis.
PMCID: PMC3676600  PMID: 23334705
2.  Collecting a comprehensive evidence base to monitor fracture rehabilitation: A case study 
World Journal of Orthopedics  2013;4(4):259-266.
AIM: To determine the feasibility and potential role of combining radiostereometric analysis (RSA), gait analysis and activity monitoring in the follow-up of fracture patients.
METHODS: Two patients with similar 41B3 tibial plateau fractures were treated by open reduction internal fixation augmented with impaction bone grafting and were instructed to partial weight bear to 10 kg for the first six postoperative weeks. Fracture reduction and fixation were assessed by postoperative computer tomographic (CT) scanning. Both patients had tantalum markers inserted intra-operatively to monitor their fracture stability during healing using RSA and differentially loaded RSA (DLRSA) at 6 and 12 wk postoperatively. Gait analyses were performed at 1, 2, 6, and 12 wk postoperatively. Activity monitors were worn for 4 wk between the 2 and 6 wk appointments. In addition to gait analysis, knee function was assessed using the patient reported Lysholm scores, and doctor reported knee range of motion and stability, at 6 and 12 wk postoperatively.
RESULTS: There were no complications. CT demonstrated that both fractures were reduced anatomically. Gait analysis indicated that Patient 1 bore weight to 60% of body weight at 2 wk postoperative and 100% at 6 wk. Patient 2 bore weight at 10% of body weight to 6 wk and had very low joint contact forces to that time. At 12 wk however, there was no difference between the gait patterns in the two patients. Patient 1 increased activities of moderate-vigorous intensity from 20 to 60 min/d between 2 and 6 postoperative weeks, whereas Patient 2 remained more stable at 20-30 min/d. The Lysholm scores were similar for both patients and did not improve between 6 and 12 wk postoperatively. DLRSA examination at 12 wk showed that both patients were comfortable to weight bear to 80 kg and under this weight the fractures displaced less than 0.4 mm. RSA measurements demonstrated over time fracture migrations of less than 2 mm in both cases. However, Patient 2, who followed the postoperative weight bearing instructions most closely, displaced less (0.3 mm vs 1.6 mm).
CONCLUSION: This study demonstrates the potential of using a combination of RSA, gait analysis and activity monitoring to obtain a comprehensive evidence base for postoperative weight bearing schedules during fracture healing.
PMCID: PMC3801245  PMID: 24147261
Radiostereometric analysis; Gait analysis; Activity monitoring; Rehabilitation protocols; Lower limb trauma
3.  Emerging Ideas: Soft Tissue Applications of Radiostereometric Analysis 
Currently, the movement that occurs at the site of soft tissue repair cannot be measured accurately in vivo. Radiostereometric analysis (RSA) is the gold standard for measuring movement between two skeletal segments in vivo but its application to studying soft tissue migration has been limited by the unknown stability of tantalum beads in tendons and ligaments and their ability to define rigid bodies in these structures.
RSA can be used to accurately measure movement between two tendon segments or between a bony and a tendon segment in vivo.
Methods of Study
The stability of tantalum beads and the ability to use such beads to define rigid bodies in some soft tissues will be investigated in animal models of tendon repair. Several tantalum bead insertion techniques will be tested using in vivo RSA measurements of bead movement followed by morphologic studies of the repaired tendon and soft tissue reaction to the tantalum beads.
RSA performed on tantalum beads encapsulated in muscles and tendons could be a powerful new tool to study the in vivo motion at the site of soft tissue repairs, for instance, the tendon gap formation. If RSA could be used to evaluate the efficiency of different tendon repair techniques in vivo, enhanced rehabilitation protocols could be developed and complications associated with prolonged protection or failure of repair could be reduced.
PMCID: PMC3069255  PMID: 21104355
4.  Early migration characteristics of a hydroxyapatite-coated femoral stem: an RSA study 
International Orthopaedics  2009;35(4):483-488.
Measurement of early stem subsidence can be used to predict the likelihood of long-term femoral component loosening and clinical failure. Data that examines the early migration pattern of clinically proven stems will provide clinicians with useful baseline data with which to compare new stem designs. This study was performed to evaluate the early migration pattern of a hydroxyapatite-coated press-fit femoral component that has been in use for over ten years. We enrolled 30 patients who underwent THA for osteoarthritis. The median age was 70 years (range, 55–80 years). Patients were clinically assessed using the Harris hip score. Radiostereometric analysis was used to evaluate stem migration at three to four days, six months, one year and two years. We observed a mean subsidence of 0.73 mm at six months, 0.62 mm at one year and 0.58 mm at two years and a mean retroversion of 1.82° at six months, 1.90° at one year and 1.59° at two years. This data suggests that subsidence is confined to the first six months after which there was no further subsidence. The results from this study can be compared with those from novel cementless stem designs to help predict the long-term outcome one may expect from new cementless stem designs.
PMCID: PMC3066322  PMID: 20012862
5.  Second-generation Highly Cross-linked X3™ Polyethylene Wear: A Preliminary Radiostereometric Analysis Study 
First-generation highly cross-linked polyethylene liners have reduced the incidence of wear particle-induced osteolysis. However, failed acetabular liners have shown evidence of surface cracking, mechanical failure, and oxidative damage. This has led to the development of second-generation highly cross-linked polyethylene, which has improved wear and mechanical properties and resistance to oxidation in vitro. Owing to its recent introduction, there are no publications describing its clinical performance.
We assessed early clinical wear of a second-generation highly cross-linked polyethylene liner and compared its clinical performance with the published results of hip simulator tests and with first-generation highly cross-linked polyethylene annealed liners.
Patients and Methods
Twenty-one patients were enrolled in a prospective cohort study. Clinical outcome and femoral head penetration were measured for 19 patients at 6 months and 1 and 2 years postoperatively.
The median proximal head penetration was 0.009 mm and 0.024 mm at 1 and 2 years, respectively. The median two-dimensional (2-D) head penetration was 0.083 mm and 0.060 mm at 1 and 2 years, respectively. The median proximal wear rate between 1 and 2 years was 0.015 mm/year.
The wear rate calculated was similar to the in vitro wear rate reported for this material; however, it was less than the detection threshold for this technique. Although longer followup is required for wear to reach a clinically quantifiable level, this low level of wear is encouraging for the future clinical performance of this material.
Level of Evidence
Level IV, therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence.
PMCID: PMC3049610  PMID: 20151231
6.  Case Report: Cementless Stem Stabilization after Intraoperative Fracture: A Radiostereometric Analysis 
We present the case of a patient with intraoperative femoral fracture during THA, which was repaired using cerclage fixation and insertion of an hydroxyapatite-coated cementless stem. The patient was evaluated postoperatively using radiostereometry during a 2-year course, and despite a large amount of subsidence and rotation, stabilization occurred and was maintained by 6 months. By evaluating the pattern of stem migration after intraoperative fracture, this case shows, even in the presence of instability, a successful clinical outcome can be achieved using an hydroxyapatite-coated cementless stem.
PMCID: PMC2816748  PMID: 19760467
7.  The accuracy and precision of radiostereometric analysis in monitoring tibial plateau fractures 
Acta Orthopaedica  2010;81(4):487-494.
Background and purpose
The application of radiostereometric analysis (RSA) to monitor stability of tibial plateau fractures during healing is both limited and yet to be validated. We therefore evaluated the accuracy and precision of RSA in a tibial plateau fracture model.
Combinations of 3, 6, and 9 markers in a lateral condyle fracture were evaluated with reference to 6 proximal tibial arrangements. Translation and rotation accuracy was assessed with displacement-controlled stages, while precision was assessed with dynamic double examinations. A comparison of error according to marker number and arrangement was completed with 2-way ANOVA models.
The results were improved using more tantalum markers in each segment. In the fracture fragment, marker scatter in all axes was achieved by a circumferential arrangement (medial, anterior, and lateral) of the tantalum markers above the fixation devices. Markers placed on either side of the tibial tuberosity and in the medial aspect of the fracture split represented the proximal tibial reference segment best. Using 6 markers with this distribution in each segment, the translation accuracy (root mean square error) was less than 37 μm in all axes. The precision (95% confidence interval) was less than ± 16 μm in all axes in vitro. Rotation, tested around the x-axis, had an accuracy of less than 0.123° and a precision of ± 0.024°.
RSA is highly accurate and precise in the assessment of lateral tibial plateau fracture fragment movement. The validation of our center's RSA system provides evidence to support future clinical RSA fracture studies.
PMCID: PMC2917573  PMID: 20465528
8.  Differentially Loaded Radiostereometric Analysis to Monitor Fracture Stiffness: A Feasibility Study 
Inability to accurately and objectively assess the mechanical properties of healing fractures in vivo hampers clinical fracture management and research. We describe a method to monitor fracture stiffness during healing in a clinical research setting by detecting changes in fracture displacement using radiostereometric analysis and simultaneously measuring applied axial loads. A method was developed for load application, positioning of the patient, and radiographic setup to establish the technique of differentially loaded radiostereometric analysis (DLRSA). A DLRSA examination consists of radiostereometric analysis radiographs taken without load (preload), under different increments of load, and without load (postload). Six patients with distal femur fractures had DLRSA examinations at 6, 12, 18, and 26 weeks postoperatively. The DLRSA method was feasible in a clinical setting. The method provides objective and quantifiable data for internally fixed fractures and may be used in clinical research as a tool to monitor the in vivo stiffness of healing femoral fractures managed with nonrigid internal fixation.
PMCID: PMC2690744  PMID: 19184262
9.  Effect of a novel interspinous implant on lumbar spinal range of motion 
European Spine Journal  2009;18(5):696-703.
Interspinous devices have been introduced to provide a minimally invasive surgical alternative for patients with lumbar spinal stenosis or foraminal stenosis. Little is known however, of the effect of interspinous devices on intersegmental range of motion (ROM). The aim of this in vivo study was to investigate the effect of a novel minimally invasive interspinous implant, InSwing®, on sagittal plane ROM of the lumbar spine using an ovine model. Ten adolescent Merino lambs underwent a destabilization procedure at the L1–L2 level simulating a stenotic degenerative spondylolisthesis (as described in our earlier work; Spine 15:571–576, 1990). All animals were placed in a side-lying posture and lateral radiographs were taken in full flexion and extension of the trunk in a standardized manner. Radiographs were repeated following the insertion of an 8-mm InSwing® interspinous device at L1–L2, and again with the implant secured by means of a tension band tightened to 1 N/m around the L1 and L2 spinous processes. ROM was assessed in each of the three conditions and compared using Cobb’s method. A paired t-test compared ROM for each of the experimental conditions (P < 0.05). After instrumentation with the InSwing® interspinous implant, the mean total sagittal ROM (from full extension to full flexion) was reduced by 16% from 6.3° to 5.3 ± 2.7°. The addition of the tension band resulted in a 43% reduction in total sagittal ROM to 3.6 ± 1.9° which approached significance. When looking at flexion only, the addition of the interspinous implant without the tension band did not significantly reduce lumbar flexion, however, a statistically significant 15% reduction in lumbar flexion was observed with the addition of the tension band (P = 0.01). To our knowledge, this is the first in vivo study radiographically showing the advantage of using an interspinous device to stabilize the spine in flexion. These results are important findings particularly for patients with clinical symptoms related to instable degenerative spondylolisthesis.
PMCID: PMC3234001  PMID: 19198894
Interspinous implant; Biomechanics; Spondylolisthesis; Lumbar spine; Kinematics

Results 1-9 (9)