Migration Of Acetabular Component Research Articles

Clinical and Radiographic Outcomes of Total Hip Arthroplasty With a Specific Liner in Small Asian Patients: Influence of Patient-Related, Implant-Related, and Surgical Factors on Femoral Head Penetration

BackgroundWe evaluated the clinical and radiographic outcomes, including femoral head penetration, of total hip arthroplasty performed using a specific polyethylene (PE) liner in small Asian patients at 10 years after the index surgery. In addition, we investigated whether femoral head penetration was affected by patient-related, implant-related, and surgical factors. MethodsBetween August 2002 and June 2005, for cementless primary total hip arthroplasty, we used acetabular PE liners that were manufactured from GUR 1050 resin, machined from isostatic compression-molded bar stock, and sterilized with a gamma ray irradiation in argon gas. We assessed 82 hips in 78 patients who received these liners. ResultsThe mean Harris hip score improved from 41.0 preoperatively to 84.5 at 10 years postoperatively. Periprosthetic osteolysis was observed in 7 hips (9.8%). No acetabular component migration was detected, and no revision surgery was performed 10 years postoperatively. The mean steady-state wear rate was 0.031 mm/y, which was lower than the wear rate for other conventional PE liners of the previous studies. Among the patient-related, implant-related, and surgical factors, sex was significantly associated with the mean steady-state wear rate, with a higher rate in male patients than in female patients. ConclusionPE acetabular liners used in small Asian patients show similar clinical outcomes and reduced wear compared with those of other liners. In addition, sex is significantly associated with the mean steady-state wear rate, and the steady-state wear rate is higher in male patients than in female patients.

Local bisphosphonate reduces migration and formation of radiolucent lines adjacent to cemented acetabular components.

Post-operative migration of cemented acetabular components as measured by radiostereometric analysis (RSA) has a strong predictive power for late, aseptic loosening. Also, radiolucent lines predict late loosening. Migration has been reduced by systemic bisphosphonate treatment in randomised trials of hip and knee arthroplasty. Used as a local treatment, a higher local dose of bisphosphonate can be achieved without systemic exposure. We wished to see if this principle could be applied usefully in total hip arthroplasty (THA). In this randomised placebo-controlled, double-blinded trial with 60 participants, we compressed gauze soaked in bisphosphonate solution (ibandronate) or saline against the acetabular bone bed immediately before cementing the acetabular component. RSA, classification of radiolucent lines, the Harris Hip Score (HHS) and the Western Ontario McMasters Universities Osteoarthritis Index (WOMAC) were carried out at three-, six-, 12-, and 24-month follow-up. Migration of the cemented acetabular component relative to the pelvis was reduced by movement almost half in the ibandronate group, when measured as maximum total point or as movement of the femoral head (p = 0.001 and 0.004, respectively). Radiolucent lines after one year were classified as absent, partial or complete, and correlated with treatment (rho 0.37; p = 0.004). Only three of 30 patients in the ibandronate group had complete lines, compared with 13 of 28 in the placebo group (p = 0.002). There were no significant effects on HHS or WOMAC score. Considering the power of RSA to predict loosening of cemented acetabular components, and the likelihood that radiolucent lines indicate risk of loosening, these data suggest that local treatment with a bisphosphonate can reduce the risk of late aseptic loosening. Cite this article: Bone Joint J 2017;99-B:317-24.

High proximal migration in cemented acetabular revisions operated with bone impaction grafting; 47 revision cups followed with RSA for 17 years.

Bone impaction grafting is a biologically and mechanically appealing option in acetabular revision surgery, allowing restitution of the bone stock and restoration of the biomechanics. We analysed differences in proximal migration of the revision acetabular components when bone impaction grafting is used together with a cemented or an uncemented cup. 43 patients (47 hips), revised due to acetabular loosening and judged to have less than 50% host bone-implant contact were included. The hips were randomised to either an uncemented (n = 20) or a cemented (n = 27) revision cup. Radiostereometry and radiography was performed postoperatively, at 3 and 6 months, 1, 2, 3, 5, 7, 10 and 13 and 17 years postoperatively. Clinical follow-up was performed at 1, 2 and 5 years postoperatively and thereafter at the same interval as in the radiographic follow-up. There were no differences in the base line demographic data between the 2 groups. At the last follow-up (17 years) 14 hips (10 cemented, 4 uncemented) had been re-revised due to loosening. 3 additional cups (1 uncemented and 2 cemented) were radiographically loose. There was a higher early proximal migration in the cemented cups. Cups operated on with cement showed a higher early migration measured with RSA and also a higher number of late revisions. The reason for this is not known, but factors such as inclusion of cases with severe bone defects, use of smaller bone chips and issues related to the impaction technique might have had various degrees of influence.

Same Day Total Hip Arthroplasty Performed at an Ambulatory Surgical Center: 90-Day Complication Rate on 549 Patients

BackgroundThere is an increasing interest in outpatient total hip arthroplasty (THA), as there are perceived benefits to the patient, insurer, and overall healthcare system. However, the safety of outpatient total joint arthroplasty has not been studied. MethodsFive hundred forty-nine patients who underwent mini-posterior THA at a freestanding independent ambulatory surgical center (ASC) were reviewed. All patients were discharged to home on the day of surgery. ResultsThe average age of the patients was 54.4 years (range 27-73). The average American Society of Anesthesiologists score was 1.6 (range 1-3). Of the 549 patients, 3 (0.5%) admitted from the surgery center to our local hospital. One patient was admitted for pain control after failing to disclose his long-term high-dose narcotic dependence, one patient was admitted for an acetabular component migration identified on postoperative x-ray, and one patient was admitted for hypotension, bradycardia, and an acute polyarthralgia exacerbation. An additional patient was seen 2 days after surgery in a local emergency department for oversedation secondary to narcotics and later discharged to home. ConclusionOutpatient THA at an ASC is safe and effective when performed on the appropriately indicated patient. There were 4 visits to the hospital within 2 days of surgery. Only 1 was related to medical events, 2 were pain control and/or medication-related and the final was technique-related.Known orthopedic complications including infection, dislocation, and deep vein thrombosis appear consistent with the literature for a series of this size.Same day discharge THA in an ASC is safe and reproducible.

The cementless fibre mesh coated anatomic femoral stem: 12 to 23 years clinical and radiological outcome study.

The aim of this study was to assess the long-term clinical and radiographic results, and possible reasons for failure of two versions of the cementless fibre-metal coated anatomical femoral component. 99 patients (119 hips) were evaluated at an average of 16.7 years (range, 12 to 23 years) following primary THA. Developmental dysplasia of the hip (DDH) was the most common pathology (n = 37 hips). Structural femoral head autografts were used to increase cup coverage in 23 hips. Vertical and horizontal migration of acetabular and femoral components and osteolysis around the components were assessed using conventional radiolography. For clinical evaluation, the Harris Hip Score and Merle D'Aubigne scale were used preoperatively and at final follow up. Implant survival was calculated according to the Kaplan-Meier survivorship analysis and failure was defined as component's revision for any reason. The cumulative survival rate of the anatomical femoral stem was 64.2% at 19.8 years, with a worst-case scenario of 41%. The cumulative survival rate of the total hip arthroplasty was 51% at 19.8 years, with a worse case scenario of 38%. The mean preoperative Harris Hip Score of 42 points improved to 88 postoperatively. The mean preoperative Merle D'Aubigne functional score of 3.5 improved to 5.2 points at the time of final follow-up. In 36 hips (41.6%) osteolysis was found in Gruen zone 1, proximal to the ingrowth surface. The cementless fibre-metal coated anatomical femoral component showed favourable long-term clinical and radiological results. Proximal coating seems to protect bone implant interface from wear debris induced osteolysis during the second decade.

Revision of migrated pelvic acetabular components in THA with or without vascular involvement

Purpose. The literature describes a high rate of mortality in cases of intrapelvic acetabular component migration, which is a rare but serious complication. Our aim is to establish and propose a treatment protocol according to our results and experience. Material and Methods. We performed eight (8) total hip revisions with acetabular cup migration between 2006 and 2012. A vascular graft was needed in four (4) of these cases. Two (2) cases were revisions after a spacer for infected arthroplasties. The protocol included the following: X-Ray examination (frontal and lateral views), CT angiography, a biological evaluation, a suitable pre-operative plan, at least six (6) units of blood stock, an experienced anesthesiologist, an experienced surgical team that included a vascular surgeon and a versatile arsenal of revision prostheses, bone grafts and vascular grafts. The anterolateral approach was generally used for hip revisions and the retroperitoneal approach in the dorsal decubitus position was used when vascular risk was involved. Results: The acetabular defect was reconstructed using bone grafts and tantalum revision cups in 4 cases, Burch-Schneider cages in 2 cases, a Kerboull ring in 1 case and a cementless oblong cup (Cotyle Espace) in 1 case. In 4 cases, an iliac vessel graft procedure was conducted by the vascular surgeon. All patients survived the revision procedures and returned regularly for subsequent check-ups, during which they did not show any septic complications. Conclusions: Intrapelvic acetabular cup migration is a rare but serious complication that can occur after total hip arthroplasty in either septic or aseptic cases. An experienced, multidisciplinary team of surgeons should be involved in planning and conducting such complicated revisions.

Does cup-cage reconstruction with oversized cups provide initial stability in THA for osteoporotic acetabular fractures?

The incidence of acetabular fractures in osteoporotic patients is increasing. Immediate total hip arthroplasty (THA) has potential advantages, but achieving acetabular component stability is challenging and, at early followup, reported revision rates for loosening are high. This study measured acetabular component stability and the initial surface contact achieved between the acetabular component and unfractured region of the pelvis after THA using an oversized acetabular component and cup-cage reconstruction. Between November 2011 and November 2013, we treated 40 acute acetabular fractures in patients older than 70 years of age. Of these, 12 (30%) underwent immediate THA using an oversized acetabular component with screws inserted only into the ilium and a cup-cage construct. Postoperatively all patients were mobilized without weightbearing restrictions. Indications for immediate THA after acetabular fractures were displaced articular comminution deemed unreducible. Eleven of the 12 were prospectively studied to evaluate the initial stability of the reconstructions using radiostereometric analysis. One of the patients died of a pulmonary embolism after surgery, and the remaining 10 (median age, 81 years; range, 72-86 years) were studied. Of these, five were analyzed at 1 year and five were analyzed at 2 years. Acetabular component migration was defined as acceptable if less than the limits for primary THA that predict later loosening (1.76 mm of proximal migration and 2.53° of sagittal rotation). The contact surface between the acetabular component and ilium in direct continuity with the sacroiliac joint, and the ischium and pubis in direct continuity with the symphysis pubis, was measured on postoperative CT scans. At 1 year the median proximal migration was 0.83 mm (range, 0.09-5.13 mm) and sagittal rotation was 1.3° (range, 0.1°-7.4°). Three of the 10 components had migration above the suggested limits for primary THA at 1 year postoperatively. The contact surface achieved at surgery between the acetabular component and pelvis ranged from 11 to 17 cm(2) (15%-27% of each component). The majority of acetabular components in this cohort were stable despite the small contact surface achieved between the component and pelvic bone. Three of 10 migrated in excess of the limits that predict later loosening in primary THA but it remains to be seen whether these limits apply to this selected group of frail osteoporotic patients. We continue to use this technique routinely to treat patients with the same indications, but since the analysis of these data we have added screw fixation of the acetabular component to the ischial tuberosity and the superior pubic ramus. Level IV, therapeutic study.

Unipolar versus Bipolar Hemiarthroplasty for Displaced Femoral Neck Fractures in the Elderly: Is There a Difference?

Hip hemiarthoplasties are commonly performed for displaced femoral neck fractures. Considerable differences of opinion exists regarding the choice between unipolar and bipolar designs. The main theoretical advantage of a bipolar over a unipolar prosthesis is the reduction of acetabular erosion due to movement taking place within the implant rather than at the acetabular implant interface. It is thus hypothesised that bipolar prostheses lead to better long-term functional outcomes with less complications. In this study, we aimed to compare unipolar (Moore's) and bipolar hemiarthroplasty looking specifically for differences in 1) pain and functional hip scores; 2) rates of acetabular erosion, component migration and revision surgery; and 3) rates of postoperative morbidity. Inclusion criteria were 1) age more than or equal to 65 years; 2) displaced femoral neck fracture of non-pathologic origin; 3) normal cognitive function; 4) ambulatory with or without assistive devices prior to the fracture; and 5) treated with a primary prosthetic replacement. Of the 193 patients that were available for review, 118 were in the Moore's group and 75 in the bipolar group. Postoperatively, patients were assessed with regards to pain, satisfaction, Modified Harris hip score and Oxford hip score. Standard anteroposterior pelvis and lateral hip radiographs were obtained at regular intervals. These were analysed specifically with regards to acetabular erosion and component migration. There was no significant difference between a Moore's and a bipolar prosthesis regarding hip pain, functional hip scores, rates of acetabular erosion, component migration, revision surgery and complications rates. Use of the more expensive bipolar prosthesis in elderly and premorbidly ambulant patient is not justified.

Classifications In Brief: Paprosky Classification of Acetabular Bone Loss

Classifications In Brief: Paprosky Classification of Acetabular Bone Loss

Coloarticular Fistula Associated with a Pseudotumor After Metal-on-Metal Hip Resurfacing Arthroplasty: A Case Report.

Fistula formation between the bowel and an implant following hip arthroplasty is rare1-11. Documented predisposing conditions include pelvic radiation therapy, long-term corticosteroid use, pelvic migration of the hip arthroplasty acetabular component, diverticulitis, and Crohn disease2,4-7,9-13. To our knowledge, we report the first known coloarticular fistula associated with a pseudotumor after a metal-on-metal hip resurfacing arthroplasty. The patient was informed that data concerning the case would be submitted for publication, and she provided consent. A fifty-nine-year-old woman with a six-week history of pain in the left lower quadrant of the abdomen that radiated to the left groin and thigh presented to our institution. She had no change in bowel habit and no history of febrile illness. The left hip had been painful for about six months. She had undergone a total abdominal hysterectomy and bilateral salpingo-oophorectomy and omentectomy for a left ovarian tumor eighteen months earlier. The histopathology report had revealed a borderline mucinous tumor confined to the left ovary. The history also included right and left Birmingham hip resurfacing (Smith & Nephew, Warwick, United Kingdom) arthroplasty for osteoarthritis, performed nine and five years earlier, respectively. These resurfacing arthroplasties, which were performed at a different institution, had no intraoperative or postoperative complications. On examination, there was a nontender mass in the left iliac fossa. Left hip movement was considerably restricted by pain. A radiograph of the pelvis showed bilateral Birmingham hip resurfacing without evidence of loosening (Fig. 1). A computed tomography (CT) scan of the thorax, abdomen, and pelvis was obtained. This showed a 6 × 7-cm complex heterogeneous mass involving the mid and distal portion of the left psoas muscle (Fig. 2). The left external iliac vein and artery were displaced by this lesion but did …

Cemented polyethylene and cementless porous-coated acetabular components have similar outcomes at a mean of seven years after total hip replacement

The aim of this prospective randomised study was to compare the clinical and radiological results of a cemented all-polyethylene Ultima acetabular component with those of a cementless porous-coated acetabular component (PFC) following total hip replacement (THR). A total of 287 patients received either a polyethylene acetabular component (groupA) or a cobalt-chromium porous-coated component (group B) with an identical cemented femoral component and 28 mm cobalt-chromium head, thus making it the largest study of its type. Patients were evaluated radiologically and clinically using the Harris hip score (HHS). Group A comprised 183 patients (73 male, 110 female) with a mean age of 71.3 years (55 to 89). Group B comprised 104 patients (48 male, 56 female) with a mean age of 69.8 years (56 to 89). A total of 16 patients (13 in Group A, three in Group B) did not have post-operative data for analysis. The mean follow-up in group A was 7.52 years (0.4 to 15.0) and in Group B 7.87 years (0.5 to 14.0). At final follow-up the mean HHS was similar between groups A and B (74.5 (25 to 100) and 78.0 (37 to 100), respectively; p = 0.068). The total number of revisions for any cause was 28, 17 of which were in group A and 11 in group B. The ten-year survivorship was 86.8% (95% confidence interval (CI) 78.4 to 92.1) and 89.2% (95% CI 78.3 to 94.8) for groups A and B, respectively (log-rank p-value = 0.938). A total of 20 cemented and two cementless acetabular components had evidence of acetabular radiolucencies or acetabular component migration at last follow-up (p = 0.001). These results indicate that patients with a cemented all-polyethylene and cementless porous-coated polyethylene lined acetabular component have similar long-term clinical outcomes.

Three-Year Prospective Clinical and Radiological Results of a New Flexible Horseshoe Acetabular Cup

We report the three-year results of a new flexible, horseshoe-shaped acetabular cup, with a carbon fibre reinforced polyetheretherketone (CFR-PEEK) bearing surface. The 3 mm thick composite cup is designed to conserve acetabular bone stock and reproduce a near-physiological stress distribution to the adjacent bone. The cup is intended to articulate against a large diameter ceramic femoral head to produce a low-wear bearing couple that generates minimal wear debris. A prospective, two-centre clinical study of the MITCH PCR cup was started in January 2007, to verify its safety and performance. Twenty-five MITCH PCR cups were implanted by three surgeons. There were 12 men and 13 women, with a mean age of 67.9 years (range 57.4 to 74.9). The mean Oxford hip score improved from 19.6 (SD 7.5) preoperatively to 43.5 (SD 7) at 3 years. The mean Harris hip score improved from 52.9 (SD 7) to 91.4 (SD 13.8) and the Euroqol-5D score increased from 62.6 (SD 18.4) to 82.8 (SD 19). One revision of the acetabular cup was undertaken at 21 months for squeaking. This has been investigated and modification of the articular geometry has resolved the problem, on in-vitro testing. Radiological analysis showed good early osseointegration of the MITCH PCR cup. However at three years, five cases of acetabular component migration and calcar resorption were observed. Three patients have subsequently undergone revision of the acetabular cup, at 41, 42 and 50 months respectively. The cause of the osteolysis is the subject of on-going investigation.

The influence of acetabular inclination angle on the penetration of polyethylene and migration of the acetabular component

In this prospective study we studied the effect of the inclination angle of the acetabular component on polyethylene wear and component migration in cemented acetabular sockets using radiostereometric analysis. A total of 120 patients received either a cemented Reflection All-Poly ultra-high-molecular-weight polyethylene or a cemented Reflection All-Poly highly cross-linked polyethylene acetabular component, combined with either cobalt-chrome or Oxinium femoral heads. Femoral head penetration and migration of the acetabular component were assessed with repeated radiostereometric analysis for two years. The inclination angle was measured on a standard post-operative anteroposterior pelvic radiograph. Linear regression analysis was used to determine the relationship between the inclination angle and femoral head penetration and migration of the acetabular component. We found no relationship between the inclination angle and penetration of the femoral head at two years' follow-up (p = 0.9). Similarly, our data failed to reveal any statistically significant correlation between inclination angle and migration of these cemented acetabular components (p = 0.07 to p = 0.9).

Improving the Accuracy of Acetabular Component Orientation: Avoiding Malposition

Improper acetabular component orientation negatively affects the outcome of total hip arthroplasty through increasing dislocation rates, component impingement, bearing surface wear, and the number of revision surgeries. Leg length, hip biomechanics, pelvic osteolysis, and acetabular component migration are also affected by malposition. With conventional techniques, numerous variables, such as patient size, deformity and/or position, and decreased visualization, contribute to inter- and intrasurgeon acetabular component variability during surgery regardless of surgeon experience and practice volume. New acetabular component implantation techniques, such as patient-specific morphology, that incorporate anatomic landmarks may provide more accurate and individualized target zones. These techniques, coupled with the use of quantitative technology such as computer-aided navigation, may improve the precision of acetabular component placement.

A Manoeuvre to Facilitate Acetabular Component Retrieval following Intra-Pelvic Migration

Acetabular component extraction following intra-pelvic migration of a total hip replacement can be hazardous to pelvic tissues, vessels and organs. Acetabular component removal is usually performed using an abdominal retroperitoneal approach. We describe a closed reduction of the acetabular component using a traction/manipulation technique. This manoeuvre can be used in cases of intra-pelvic acetabular component migration where the surface contour of the acetabular component is uncomplicated and an interposition membrane separates the implant from adjacent intra-pelvic structures.

Long-term radiographic results of the Saint Nabor non-cemented acetabular component

Between August 1985 and December 1987, a total of 303 total hip arthroplasties were performed in 275 patients using the Saint Nabor press-fit acetabular component. Radiographic evaluations were performed at an average of 18.1 years for 95 hips in 86 patients. Migration analyses of the femoral and acetabular components were also performed. Seventeen patients had early postoperative complications: aseptic loosening in 1 patient, luxation in 16 patients. Twenty-five patients (25 hips) had late postoperative complications involving aseptic loosening: 21 femoral components, 2 acetabular components, and 2 cases of both components. The average acetabular component migration in the horizontal direction was 1.71 mm and was 0.31 mm in the vertical direction. The Kaplan–Meier survival rate of the acetabular component at 19 years was 95%.

Disassembly of Cemented Bipolar Prothesis of the Hip

A case of spontaneous disassembly of a bipolar prothesis after hemiarthroplasty due to a dislocation of the hip in an 85-year-old woman is presented, which is a rare complication. Open reduction was performed and original components were used because there was no acetabular and polyethylene linear erosion. The result was satisfactory at final follow-up (36 months), and radiographs showed no acetabular erosion, polyethylene wear, femoral subsidence, or component migration. To date, only 6 cases of disassembly of bipolar components have been reported in the English literature. In almost all cases, the injury was caused without any significant force, as only 2 patients reported a fall. Most were elderly women. We classified them into 3 types according to the etiology: iatrogenic disassembly (type I), erosion of the polyethylene liner (type II), and spontaneous disassembly (type III). Spontaneous disassembly is uncommon and seemed to be caused by a failure of the locking mechanism between the head components, resulting in an intraacetabular dislocation. Open reduction is required and revision surgery is judged by the locking mechanism between components. When there is no acetabular and polyethylene linear erosion, we believe that the original components can be used again. Although the potential disadvantage of bipolar systems shown here does not outweigh the advantages, we should rethink the status of bipolar hemiarthroplasty.

High-precision measurements of cementless acetabular components using model-based RSA: An experimental study

Background In RSA, tantalum markers attached to metal-backed acetabular cups are often difficult to detect on stereo radiographs due to the high density of the metal shell. This results in occlusion of the prosthesis markers and may lead to inconclusive migration results. Within the last few years, new software systems have been developed to solve this problem. We compared the precision of 3 RSA systems in migration analysis of the acetabular component.Material and methods A hemispherical and a nonhemispherical acetabular component were mounted in a phantom. Both acetabular components underwent migration analyses with 3 different RSA systems: conventional RSA using tantalum markers, an RSA system using a hemispherical cup algorithm, and a novel modelbased RSA system.Results We found narrow confidence intervals, indicating high precision of the conventional marker system and model-based RSA with regard to migration and rotation. The confidence intervals of conventional RSA and model-based RSA were narrower than those of the hemispherical cup algorithm-based system regarding cup migration and rotation.Interpretation The model-based RSA software combines the precision of the conventional RSA software with the convenience of the hemispherical cup algo-rithm-based system. Based on our findings, we believe that these new tools offer an improvement in the measurement of acetabular component migration.

Cementless acetabular component inserted without screws – the effect of immediate weight-bearing

Seventy-four total hip arthroplasties using a cementless acetabular component without holes for supplemental screw fixation were evaluated during a follow-up period of 2-6 years. Immediate full weight-bearing walking was allowed after the operation. A comparison of anteroposterior (AP) radiographs of the pelvis taken in the early postoperative period and when the patient returned for the final follow-up revealed that there was no migration of the acetabular component. Based on the early postoperative radiographs and using the zones described by Martell et al. [J Bone Joint Surg (1993) 75-A:554-571], gaps between the acetabular component and the bone occurred at zone A2 in three hips (4.1%), at zone B1 in 12 hips (16.2%), and at zone B2 in seven hips (9.5%). The final follow-up radiographs, however, revealed only one hip to have a gap of 1.1 mm at zone B2; the majority of the gaps had disappeared. The results suggest that immediate weight-bearing walking did not result in the migration of the cementless acetabular component.

Model Studies on Acetabular Component Migration in Total Hip Arthroplasty using Ct and a Semiautomated Program for Volume Merging

Validation of a non-invasive CT method for detection of acetabular cup migration after total hip arthroplasty in a phantom study. 26 CT examinations were obtained of a pelvic model while altering the position of the acetabular cup. Using a previously described program for volume merging, the pelvi in different examinations were fused and the 3D alterations of the position of the acetabular cup were evaluated visually and numerically and correlated to direct measurements on the model. Visually, two independent examiners differentiated between 0, 1 and 2 to 3 mm migration with 100% specificity and sensitivity. Numerically, the mean error over all cases between model and CT measurements was 0.04 mm (SD +/- 0.33). The mean absolute error between model and CT data was 0.26 mm (SD +/- 0.19). Intra- and interobserver 95% accuracy and repeatability limits were below 0.5/0.7 mm, respectively. No significant interobserver difference occurred. The data were normally distributed and not dependent on observer. The accuracy of this non-invasive method out-performs routine plain radiography. The method gives both visual and numerical correlates to migration and can be used in clinical practice.