Purpose: Efforts to decrease polyethylene wear have lead to advances in polyethylene and counter-face technology for total hip replacement. In particular, the use of highly cross-linked polyethylene (XLPE) and more recently, oxidized zirconium (Oxinium) heads, have demonstrated significant in-vitro improvements in THR wear. This study reports on the early clinical performance and wear (measured with RSA) of an randomized controlled trial (RCT) comparing Oxinium and CoCr heads on XLPE and conventional polyethylene (CPE).

Method: Forty patients were enrolled in a RCT and stratified to receive either an Oxinium (Ox) or CoCr head against either XLPE or CPE (ie 10 patients in each group). All patients had otherwise identical THRs and had tantalum beads inserted in the pelvis and polyethylene for wear analysis. There were no significant differences between groups with respect to patient demographics and the average age was 68 years (range 57–76) at index procedure. RSA wear analysis was performed immediately post-op, at six weeks, three and six months and then at one and two years. All patients are a minimum of four years post-op (average 4.6, range 4 – 5.8). Patients were followed prospectively using validated clinical outcome scores (WOMAC, SF-12, Harris Hip scores) and radiographs.

Results: All health-related outcomes were significantly improved from pre-operative with a mean Harris Hip score and WOMAC at last follow-up of 90.9 and 80.2, respectively. Total 3D femoral head penetration at two years for each group were the following: CoCrXLPE (0.068±0.029mm); OxXLPE (0.115±0.038mm); CoCrCPE (0.187±0.079mm); and OxCPE (0.242±0.088mm). Thus, OxCPE was significantly higher than OxXLPE and CoCrXLPE but not CoCrCPE (p=0.001, p> 0.0001 and p=0.094, respectively). In other words, head penetration was higher with CPE compared to XLPE but there was no significant difference between Ox and CoCr heads. Similarily, regardless of head type (ie combining similar poly types), there was a significant difference in 3D head penetration at two years between CPE and XLPE ( CPE 0.213±0.086; XLPE 0.093±0.041, p> 0.0001).

Conclusion: The early results of this RCT, using RSA as the wear analysis tool, indicate a significant improvement in wear with XLPE compared to CPE. However, it failed to show a clear advantage to the use of Oxinium over CoCr against either polyethylene. Longer follow-up is required to determine steady-state wear rates (after bedding-in) and allow comparison between bearing groups.

Purpose: This study develops and validates a technique to quantify polyethylene wear in tibial inserts using micro-computed tomography (micro-CT), a nondestructive high resolution imaging technique that provides detailed images of surface geometry in addition to volumetric measurements.

Method: Six unworn and six wear-simulated Anatomic Modular Knee (DePuy Inc, Warsaw, IN) tibial inserts were evaluated. Each insert was scanned three times using micro-CT at a resolution of 50 μm. The insert surface was reconstructed for each scan through automatic segmentation and the insert volume was calculated. Gravimetric analysis was also performed for all inserts, and the micro-CT and gravimetric volumes were compared to determine accuracy. The utility of surface deviation maps derived from micro-CT was demonstrated by co-registering a worn and unworn insert. 3D deviations were measured continuously across the entire insert surface, including the articular and backside surfaces.

Results: The mean percent volume difference between the micro-CT and gravimetric techniques was 0.04% for the unworn inserts and 0.03% for the worn inserts. No significant difference was found between the micro-CT and gravimetric volumes for the unworn or worn inserts (P = 0.237 and P = 0.135, respectively). The mean coefficient of variation for volume between scans was 0.07% for both unworn and worn inserts. The map of surface deviations between the worn and unworn insert revealed focal deviations exceeding 750 μm due to wear.

Conclusion: Micro-CT provides precise and accurate volumetric measurements of polyethylene tibial inserts. Quantifiable 3D articular and backside surface deviation maps can be created from the detailed geometry provided by the technique. Compared to coordinate mapping, micro-CT provides 10 times greater surface sampling resolution (50 μm vs 500 μm) across the entire insert surface. Micro-CT is a useful analysis tool for wear simulator and retrieval studies of the polyethylene components used in total knee replacement.

Purpose: The purpose of this prospective blinded randomized control trial was to compare the stem migration of two cemented stem designs using radiostereometric analysis (RSA). This was essentially a safety study in which our hypothesis was that the newer design (CPCS, Smith and Nephew Inc) would demonstrate similar micro-motion to the well-established Exeter (Stryker) design.

Method: Thirty patients were consented and enrolled into a blinded RCT in which 15 patients received a dedicated RSA CPCS stem and 15 patients received a RSA Exeter stem. Both stems are collarless tapered polished cemented stems, the only difference being a slight lateral to medial taper with the CPCS design. Outcome measures were compared (Hip Society Score, WOMAC, SF-12). RSA analysis was conducted immediately postoperatively, at 6 weeks, 3 months, 6 months, 1 year and 2 years.

Results: No difference was found in any of the outcome measures pre-operatively or postoperatively. At 2 years, stem subsidence for the CPCS stem was approximately half that seen for the Exeter stem (0.565±0.189mm and 0.981±0.209mm respectively, p< 0.0001). In contrast, posterior (internal) rotation of the CPCS was approximately twice that of the Exeter stem (1.496±1.215° and 0.716±0.818° respectively, p=0.221). Other migration patterns were no different between stems.

Conclusion: As expected with this stem design, both stems showed some axial and rotational migration within the cement mantle. The subtle differences in design may explain the differences in migration patterns. Our data suggests that the newer CPCS design should perform well over the long-term.

Purpose: Accurate measurement of dynamic joint motion remains a clinical challenge. To address this problem, we have developed a low-dose clinical procedure using the Roentgen Single-plane Photogrammetric Analysis (RSPA) technique. A validation study was performed in a clinical setting, using a conventional digital flat-panel radiography system.

Method: To validate the technique, three experiments were performed: assessment of static accuracy, dynamic repeatability and measurement of effective dose. A knee joint phantom, imbedded with tantalum markers, was utilized for the experiments. Relative spatial positions of the markers were reconstructed using Radiostereometric Analysis (RSA). A digital flat-panel radiography system was used for image acquisition, and the three-dimensional pose of each segment was determined from single-plane projections by applying the RSPA technique. All images were processed using software developed in-house. To assess static accuracy, the phantom was mounted onto a three-axis translational stage and moved through a series of displacements ranging from 0 to 500 μm. Images of the phantom were acquired at each position. Accuracy was calculated by analyzing differences between reconstructed and applied displacements. To assess dynamic repeatability, the phantom was mounted on a six-axis robot, programmed to apply a flexion-extension movement to the joint. Multiple cine acquisitions of the moving phantom were acquired (30 fps, 4 ms exposure). Repeatability was calculated by analyzing the variation between motions reconstructed from repeated acquisitions. The effective dose of the procedure was measured using an ion-chamber dosimeter. The ion chamber was positioned between the phantom and x-ray source, facing the source. Entrance exposure was measured for multiple acquisitions, from which the effective dose was calculated.

Results: The accuracy determined from the static assessment was 25 μm and 450μm at the 95% confidence intervals for translations parallel and orthogonal to the image plane, respectively. Repeatability of the motion reconstructed from dynamic acquisitions was better than ± 200 μm for translations and ± 0.1 for rotations. The average effective dose for a 6 second dynamic acquisition was approximately 2μSv.

Conclusion: The proposed clinical procedure demonstrates both a high degree of accuracy and repeatability, and delivers a low effective dose.

Purpose: Accurate acetabular cup positioning is essential to successful total hip arthroplasty (THA). Intra-operative navigation of the acetabular component can optimize positioning, but often necessitates registration of the pelvis in the supine position. The majority of surgeons use the lateral position, however, which hides commonly employed registration landmarks. The purpose of this study was to identify novel anatomical landmarks for use in navigated THA from the lateral approach.

Method: We identified 156 patients that underwent pelvic CT scans for non-orthopaedic reasons from which 60 patients (mean age 62 years; 30 males, 30 females) were included in the study. CT scans were analyzed with sophisticated software (region grow, isosurface creation, and geometry overlay features). Saved coordinates from each scan were inputted into the program MATLAB (Mathworks, Natick, MA), v7.0, on a Macintosh-based workstation. A code was created to be able to calculate the normal vector for both planes and then calculate the angle formed between the normal vectors. The anterior plane (pubic tubercle (PT) and anterior superior iliac spine (ASIS)) was defined in addition to a series of lateral planes by retaining the ipsilateral PT and ASIS from the anterior plane, plus a variable third landmark. Angles obtained were those between the anterior and lateral planes. Angle conversions between the planes were analyzed using a paired t-test with a p-value of < 0.05 accepted as significant.

Results: The list of landmarks acquired included those used for supine registration (PT and ASIS) in addition to: posterior superior iliac spine (PSIS); posterior inferior iliac spine, (PIIS); ischial tuberosity (IT); tuber-culum of the iliac crest (TIC); and a line drawn along the outer lip of the iliac crest. The angle between the anterior plane and the novel lateral planes did not show a significant level of variance for two of the proposed lateral planes (P< 0.05).

Conclusion: An imageless navigation system in THA that can be accurately employed in the lateral position will benefit many surgeons. The invariance in angle calculations for the lateral planes calculated using the PSIS and the TIC suggest that they could be novel pelvic landmarks for lateral plane registration.

This was a safety study where the hypothesis was that the newer-design CPCS femoral stem would demonstrate similar early clinical results and micromovement to the well-established Exeter stem. Both are collarless, tapered, polished cemented stems, the only difference being a slight lateral to medial taper with the CPCS stem. A total of 34 patients were enrolled in a single-blinded randomised controlled trial in which 17 patients received a dedicated radiostereometric CPCS stem and 17 a radiostereometric Exeter stem. No difference was found in any of the outcome measures pre-operatively or post-operatively between groups. At two years, the mean subsidence for the CPCS stem was nearly half that seen for the Exeter stem (0.77 mm (−0.943 to 1.77) and 1.25 mm (0.719 to 1.625), respectively; p = 0.032). In contrast, the mean internal rotation of the CPCS stem was approximately twice that of the Exeter (1.61° (−1.07° to 4.33°) and 0.59° (0.97° to 1.64°), respectively; p = 0.048). Other migration patterns were not significantly different between the stems. The subtle differences in designs may explain the different patterns of migration.

Comparable migration with the Exeter stem suggests that the CPCS design will perform well in the long term.

Isolated liner exchange in revision total hip arthroplasty for the treatment of polyethylene wear is an increasingly common surgical procedure. Twenty-four hips underwent this procedure via the direct lateral approach and were prospectively followed clinically and radiographically. Accessible osteolytic lesions were curetted and bone grafted. At a mean follow-up of forty months, a significant clinical improvement was observed. One cup collapsed into an osteolytic lesion postoperatively; all other lesions regressed. No dislocations have occurred. Isolated liner exchange via the direct lateral approach may reduce dislocation rates while avoiding the morbidity associated with the removal of well-fixed components.

The purpose of this study was to evaluate the clinical and radiographic results of isolated liner exchange in revision total hip arthroplasty (THA) for osteolysis and polyethylene wear via the direct lateral surgical approach.

Retention of well-fixed implants avoids unnecessary bone loss at revision surgery. Previous studies report a significant dislocation rate with isolated liner exchange. Revision via the direct lateral surgical approach may reduce the dislocation rate in surgery for acetabular osteolysis.

Twenty-four hips that underwent an isolated liner exchange revision procedure via the direct lateral approach were prospectively followed. Accessible osteolytic lesions were curetted and bone grafted. Harris Hip Score, WOMAC Index, and radiographic analysis were recorded. The area of osteolytic lesions was calculated using a computer imaging technique.

At mean follow-up of forty months, all except one of the osteolytic lesions had regressed in size. Mean Harris Hip scores improved from sixty-nine to eighty-three and WOMAC indices improved from thirty-seven to twenty-four. No dislocations have occurred. One cup collapsed into an osteolytic lesion postoperatively, requiring an acetabular revision procedure.

Isolated liner exchange is a promising technique that avoids the removal of well-fixed acetabular implants. The increased dislocation rate associated with revision THA may be reduced and osteolytic lesions may be debrided and bone grafted through the direct lateral approach.

Isolated liner exchange via the direct lateral approach reduces the dislocation rate in THA. Retention of wellfixed implants and bone grafting is a procedure that preserves bone stock and addresses osteolytic lesions at revision surgery.

Purpose: Achieving accurate imaging in talus neck fracture management is important. Patient outcome relies on the ability to achieve and maintain anatomic reduction. The ability to visualize the reduction postoperatively using plain radiographs or computed tomography (CT) is limited. The purpose of this study is to assess the ability of radiostereometric analysis (RSA) to detect changes in talus fracture fragment alignment using an in-vitro model. This will be compared to the use of plain radiographs and CT.

Methods: Eight cadaveric human lower extremities were used as talus fracture models. Each talus was removed from the specimen and an osteotomy was created. RSA beads were inserted into the fragments. Anatomical reduction was achieved with two 3.5 mm cortical screws. A set of plain radiographs and RSA films was obtained. The fragments were displaced in a combined varus and supination direction. The degree of displacement was measured with a Vernier caliper and the rotation measured with a protractor. The imaging sequence was repeated in addition to obtaining CT scans with three dimension reconstruction. The RSA measurements were interpreted in a blinded fashion by an experienced researcher. Two independent blinded observers measured the displacement and rotation with plain films and CT. The results from each radiographic measurements were compared using ANOVA method to the experimental values.

Results: The average difference between the RSA measurements and the experimental measurements was 5.9mm while the difference between CT scan measurements and experimental values was 2.4mm (p=0.003). The average difference in rotation was not statistically significant between the three groups.

Conclusions: CT scan provides the most accurate assessment of talar neck malunion. Unfortunately, RSA is not a viable imaging technique for assessing talar neck displacement.

Funding : Other Education Grant

Funding Parties : Lawson Health Research Institute

The timing of liner exchange for retroacetabular oste-olysis in THA remains uncertain. Liner exchange should be done before the shell becomes loose. The purpose of this study was to determine the radiographic quantity of osteolysis that will predict impending loosening of the cementless shell. Osteolytic lesions were quantified radiographically using three different measures. Implant stability was confirmed intraoperatively. Percent of shell circumference with surrounding osteolysis appears to be more predictive of cementless shell loosening than the area of osteolysis. When greater than fifty percent of the shell circumference has osteolysis on AP or lateral films, liner exchange is necessary.

The timing of liner exchange for retroacetabular osteolysis in cementless THA remains uncertain. Liner exchange should be done before the shell becomes loose.

To determine the radiographic quantity of osteolysis that will predict impending loosening of the cementless shell.

Osteolytic areas differ between loose and stable shells.

Between 1992 to 2002, seventy-one cementless shells of the same design were revised at our institution; forty-six were for aseptic retroacetabular osteolysis. Radiographs and a computer-assisted technique were used to quantify osteolytic areas and percent of shell circumference associated with lesions. Implant stability was confirmed intraoperatively.

Of twenty-six stable and twenty loose shells, the average area of osteolysis on AP radiographs showed no significant difference (Stable 591mm², Loose 630 mm², p greater than 0.05); whereas, lateral radiographs demonstrated a significant difference (Stable 546 mm², Loose 837 mm², p 0.05). The percentage of shell circumference with associated osteolysis on AP and lateral films demonstrated a significant difference (AP: Stable 60, Loose 75, p 0.042; LATERAL: Stable 49, Loose73, p 0.016). Diagnostic criteria of fifty percent shell circumference associated with osteolysis on AP films has sensitivity 1, specificity 0.27 while the same criteria on lateral films has sensitivity 0.84, specificity 0.54 for shell loosening.

Percent of shell circumference with surrounding oste-olysis appears to be more predictive of cementless shell loosening than the area of osteolysis. When greater than fifty percent of the shell circumference has osteolysis on AP or lateral films, liner exchange is necessary.

Introduction and Aims: Problem: The timing of liner exchange for retroacetabular osteolysis in THA remains uncertain. Liner exchange should be done before the shell becomes loose.

Purpose: To determine the radiographic quantity of osteolysis that will predict impending loosening of the cementless shell.

Hypothesis: Osteolytic areas differ between loose and stable shells.

Method: Between 1992 and 2002, 71 cementless shells of the same design were revised at our institution; 46 were for aseptic retroacetabular osteolysis. Radiographs and a computer-assisted technique were used to quantify osteolytic areas and percent of shell circumference associated with lesions. Implant stability was confirmed intra-operatively.

Results: Of 26 stable and 20 loose shells, the average area of osteolysis on AP radiographs showed no significant difference (Stable 591mm2, Loose 630 mm2, p greater than 0.05); whereas, lateral radiographs demonstrated a significant difference (Stable 546 mm2, Loose 837 mm2, p 0.05). The percentage of shell circumference with associated osteolysis on AP and lateral films demonstrated a significant difference (AP: Stable 60, Loose 75, p 0.042; LATERAL: Stable 49, Loose 73, p 0.016). Diagnostic criteria of 50 percent shell circumference associated with osteolysis on AP films has sensitivity 1.0, specificity 0.27, while the same criteria on lateral films has sensitivity 0.84, specificity 0.54 for shell loosening.

Conclusion: Percent of shell circumference with surrounding osteolysis appears to be more predictive of cementless shell loosening than the area of osteolysis. When greater than 50 percent of the shell circumference has osteolysis on AP or lateral films, liner exchange is necessary.

Introduction and Aims: Polyethylene wear in total hip arthroplasty (THA) is frequently associated with wellfixed cementless implants. Purpose: To evaluate the clinical and radiographic results of the isolated liner exchange (ILE) procedure in revision THA via the direct lateral surgical approach.

Method: A prospective study of 24 hips that underwent an ILE revision procedure via the direct lateral approach was conducted. Accessible osteolytic lesions were managed with curettage and bone grafting. Clinical data including Harris hip score, WOMAC Index, SF-12, and radiographic analysis were recorded. The area of osteolytic lesions was calculated using a new software program.

Results: This is the first study to our knowledge to report on the results of the ILE procedure performed via the direct lateral surgical exposure. Twenty-three patients underwent 24 revisions with an ILE. At mean follow-up of 40 months, all osteolytic lesions had regressed. Harris hip scores improved from 69 to 83. WOMAC indices improved from 37 to 24. No dislocations have occurred. Two patients have required revision. Isolated liner exchange for polyethylene wear is a promising technique that avoids the removal of well-fixed acetabular implants. The increased dislocation rate associated with this revision THA procedure may be reduced and osteolytic lesions may be successfully debrided and bone grafted through the direct lateral approach.

Conclusion: The ILE procedure, when performed via the direct lateral surgical approach, may reduce the dislocation rate commonly reported via the posterolateral exposure with this procedure. Retention of well-fixed implants and bone grafting preserves bone stock and adequately addresses osteolytic lesions at revision surgery through this exposure.

We present a new approach for the accurate reconstruction of three-dimensional skeletal positions using roentgen single-plane photogrammetric analysis (RSPA). This technique uses a minimum of three markers embedded in each segment which allow continuous, real-time, internal skeletal movement to be measured from single-plane images, provided that the precise distance between the markers is known.

A simulation study indicated that the error propagation in this approach is influenced by focus position, object position, the number of control points, the accuracy of the previous measurement of the distance between markers and the accuracy of image measurement. For reconstruction of normal movement of the knee with an input measurement error of sd = 0.02 mm, the rotational and translational differences between reconstructed and original movement were less than 0.27° and 0.9 mm, respectively.

Our results showed that the accuracy of RSPA is sufficient for the analysis of most movement of joints. This approach can be applied in combination with force measurements for dynamic studies of the musculoskeletal system.