Combined surface and volume processing for fused joint segmentation.

PURPOSE: Segmentation of rheumatoid joints from CT images is a complicated task. The pathological state of the joint results in a non-uniform density of the bone tissue, with holes and irregularities complicating the segmentation process. For the specific case of the shoulder joint, existing segmentation techniques often fail and lead to poor results. This paper describes a novel method for the segmentation of these joints. METHODS: Given a rough surface model of the shoulder, a loop that encircles the joint is extracted by calculating the minimum curvature of the surface model. The intersection points of this loop with the separate CT-slices are connected by means of a path search algorithm. Inaccurate sections are corrected by iteratively applying a Hough transform to the segmentation result. RESULTS: As a qualitative measure we calculated the Dice coefficient and Hausdorff distances of the automatic segmentations and expert manual segmentations of CT-scans of ten severely deteriorated shoulder joints. For the humerus and scapula the median Dice coefficient was 98.9% with an interquartile range (IQR) of 95.8-99.4 and 98.5% (IQR 98.3-99.2%), respectively. The median Hausdorff distances were 3.06 mm (IQR 2.30-4.14) and 3.92 mm (IQR 1.96 -5.92 mm), respectively. CONCLUSION: The routine satisfies the criterion of our particular application to accurately segment the shoulder joint in under 2 min. We conclude that combining surface curvature, limited user interaction and iterative refinement via a Hough transform forms a satisfactory approach for the segmentation of severely damaged arthritic shoulder joints.

Influence of the positioning of a cementless glenoid prosthesis on its interface micromotions.

The positioning of the glenoid component in total shoulder arthroplasty is complicated by the limited view during operation. Malalignment and/or motion of the glenoid component with respect to the bone can be a cause of, or contribute to, failure of the implant. The aim of this paper is to determine the effect of the positioning of a cementless glenoid component on the micromotions between the implant and the bone during normal loading after surgery. For this study a three-dimensional finite element model of a complete scapula with a cementless glenoid component was used. In total, eight positions of the upper arm in both abduction and anteflexion were chosen to represent the patient's arm movement postoperatively. A previously published musculoskeletal model was used to determine the joint and muscle forces on the scapula with implant in each arm position. Five different alignments of the glenoid component (neutral, anterior, inferior, posterior, and superior inclinations), two different implantation depths ('optimal' and 'deeper' implantations), and two bone qualities (healthy and rheumatoid arthritis (RA) bone) were considered. Inclinations of 10 degrees with respect to a neutral alignment did not affect the overall interface micromotions in the optimal implantation depth. However, when the implantation depth was 3 mm deeper, anterior and inferior inclinations were more favourable than a neutral alignment and other inclinations. Micromotions in RA bone were always larger than in healthy bone.

Evaluation of bone impingement prediction in pre-operative planning for shoulder arthroplasty.

In shoulder arthroplasty, malpositioning of prostheses often leads to reduced postoperative range of motion (ROM) and complications such as impingement, loosening, and dislocation. Furthermore, the risk of impingement complications increases when reverse total prostheses are used. For this purpose a pre-operative planning system was developed that enables surgeons to perform a virtual shoulder replacement procedure. The present authors' pre-operative planning system simulates patient-specific bone-determined ROM meant to reduce the risk of impingement complications and to improve the ROM of patients undergoing shoulder replacement surgery. This paper describes a validation experiment with the purpose of ratifying the clinical applicability and usefulness of the ROM simulation module for shoulder replacement surgery. The experiment was performed on cadaveric shoulders. A data connection was set up between the software environment and an existing intra-operative guidance system to track the relative positions of the bones. This allowed the patient-specific surface models to be visualized within the software for the position and alignment of the tracked bones. For both shoulders, ROM measurements were recorded and tagged with relevant information such as the type of prosthesis and the type of movement that was performed. The observed ROM and occurrences of impingement were compared with the simulated equivalents. The median deviation between observed impingement angles and simulated impingement angles was -0.30 degrees with an interquartile range of 5.20 degrees (from -3.40 degrees to 1.80 degrees). It was concluded that the ROM simulator is sufficiently accurate to fulfil its role as a supportive instrument for orthopaedic surgeons during shoulder replacement surgery.

Glenohumeral stability in simulated rotator cuff tears.

Rotator cuff tears disrupt the force balance in the shoulder and the glenohumeral joint in particular, resulting in compromised arm elevation torques. The trade-off between glenohumeral torque and glenohumeral stability is not yet understood. We hypothesize that compensation of lost abduction torque will lead to a superior redirection of the reaction force vector onto the glenoid surface, which will require additional muscle forces to maintain glenohumeral stability. Muscle forces in a single arm position for five combinations of simulated cuff tears were estimated by inverse dynamic simulation (Delft Shoulder and Elbow Model) and compared with muscle forces in the non-injured condition. Each cuff tear condition was simulated both without and with an active modeling constraint for glenohumeral stability, which was defined as the condition in which the glenohumeral reaction force intersects the glenoid surface. For the simulated position an isolated tear of the supraspinatus only increased the effort of the other muscles with 8%, and did not introduce instability. For massive cuff tears beyond the supraspinatus, instability became a prominent factor: the deltoids were not able to fully compensate lost net abduction torque without introducing destabilizing forces; unfavorable abductor muscles (i.e. in the simulated position the subscapularis and the biceps longum) remain to compensate the necessary abduction torque; the teres minor appeared to be of vital importance to maintain glenohumeral stability. Adverse adductor muscle co-contraction is essential to preserve glenohumeral stability.

Image-based RSA: Roentgen stereophotogrammetric analysis based on 2D-3D image registration.

Image-based Roentgen stereophotogrammetric analysis (IBRSA) integrates 2D-3D image registration and conventional RSA. Instead of radiopaque RSA bone markers, IBRSA uses 3D CT data, from which digitally reconstructed radiographs (DRRs) are generated. Using 2D-3D image registration, the 3D pose of the CT is iteratively adjusted such that the generated DRRs resemble the 2D RSA images as closely as possible, according to an image matching metric. Effectively, by registering all 2D follow-up moments to the same 3D CT, the CT volume functions as common ground. In two experiments, using RSA and using a micromanipulator as gold standard, IBRSA has been validated on cadaveric and sawbone scapula radiographs, and good matching results have been achieved. The accuracy was: |mu |< 0.083 mm for translations and |mu| < 0.023 degrees for rotations. The precision sigma in x-, y-, and z-direction was 0.090, 0.077, and 0.220 mm for translations and 0.155 degrees , 0.243 degrees , and 0.074 degrees for rotations. Our results show that the accuracy and precision of in vitro IBRSA, performed under ideal laboratory conditions, are lower than in vitro standard RSA but higher than in vivo standard RSA. Because IBRSA does not require radiopaque markers, it adds functionality to the RSA method by opening new directions and possibilities for research, such as dynamic analyses using fluoroscopy on subjects without markers and computer navigation applications.

A multidisciplinary hand clinic for patients with rheumatic diseases: a pilot study.

To describe the characteristics, management strategies and outcomes of patients with rheumatic diseases and complex hand function problems referred to a multidisciplinary hand clinic. Assessments (baseline and after three months of follow-up) included sociodemographic and disease characteristics and various hand function measures. The most frequently mentioned impairments and limitations of the 69 patients enrolled in the study pertained to grip, pain, grip strength, and shaking hands. Fifty-six patients received treatment advice, conservative therapy (n=39), surgery (n=12), or a combination of both (n=5). In 38 of 56 patients (68%) the recommended treatment was performed, and 33 completed the follow-up assessment. On average, patients improved, with an increase in grip strength and the Michigan Hand Outcomes Questionnaire scores reached statistical significance. Two-thirds of patients with rheumatic conditions visiting a multidisciplinary hand clinic reportedly followed the treatment advice (recommendations), with an overall trend toward a beneficial effect on hand function. To further determine the added value of a structured, multidisciplinary approach a controlled comparison with other treatment strategies is needed.

Pathological muscle activation patterns in patients with massive rotator cuff tears, with and without subacromial anaesthetics.

A mechanical deficit due to a massive rotator cuff tear is generally concurrent to a pain-induced decrease of maximum arm elevation and peak elevation torque. The purpose of this study was to measure shoulder muscle coordination in patients with massive cuff tears, including the effect of subacromial pain suppression. Ten patients, with MRI-proven cuff tears, performed an isometric force task in which they were asked to exert a force in 24 equidistant intervals in a plane perpendicular to the humerus. By means of bi-polar surface electromyography (EMG) the direction of the maximal muscle activation or principal action of six muscles, as well as the external force, were identified prior to, and after subacromial pain suppression. Subacromial lidocaine injection led to a significant reduction of pain and a significant increase in exerted arm force. Prior to the pain suppression, we observed an activation pattern of the arm adductors (pectoralis major pars clavicularis and/or latissimus dorsi and/or teres major) during abduction force delivery in eight patients. In these eight patients, adductor activation was different from the normal adductor activation pattern. Five out of these eight restored this aberrant activity (partly) in one or more adductor muscles after subacromial lidocaine injection. Absence of glenoid directed forces of the supraspinate muscle and compensation for the lost supraspinate abduction torque by the deltoideus leads to destabilizating forces in the glenohumeral joint, with subsequent upward translation of the humeral head and pain. In order to reduce the superior translation force, arm adductors will be co-activated at the cost of arm force and abduction torque. Pain seems to be the key factor in this (avoidance) mechanism, explaining the observed limitations in arm force and limitations in maximum arm elevation in patients suffering subacromial pathologies. Masking this pain may further deteriorate the subacromial tissues as a result of proximal migration of the humeral head and subsequent impingement of subacromial tissues.

Subacromial space measurement: a reliable method indicating fatty infiltration in patients with rheumatoid arthritis.

Proximal migration of the humeral head is thought to indicate fatty infiltration of the rotator cuff muscles or rotator cuff tears. We sought to evaluate the influence of these rotator cuff abnormalities on the subacromial space. Using anteroposterior radiographs, ultrasound, and computed tomography, we analyzed 54 shoulders in 29 patients with rheumatoid arthritis. The upward migration index was defined as proximal migration of the humeral head relative to its size. The mean muscle density from computed tomography images was used to indicate fatty infiltration. Fatty infiltration of the infraspinatus muscle showed the strongest correlation with proximal migration. After correcting for age, cuff tears, and rheumatoid disease, the partial correlation coefficient between both remained strong. A subdivision in proximal migration is proposed to screen for rotator cuff abnormalities. A large amount of fatty infiltration was indicated by an upward migration index less than 1.25, a medium amount by an upward migration index between 1.25 to 1.35, and a small amount by an upward migration index greater than 1.35. Measurement of proximal migration using the upward migration index provides a reliable screening method indicating fatty infiltration of the rotator cuff.

Outcome of revision surgery for failed primary Souter-Strathclyde total elbow prosthesis.

Total elbow arthroplasty is used for the treatment of the painful, destroyed elbow joint. With the increase in elbow replacement surgery in recent years, the number of revision surgeries will also increase. At our center, 236 primary Souter-Strathclyde total elbow prostheses have been placed. Twenty-four of these have been revised and were followed up for a mean of 74 months (range, 12-165 months). The postoperative clinical outcome after revision surgery can approximate the outcome after primary elbow replacement. Unfortunately, 8 elbows needed to be re-revised, 7 elbow prostheses loosened, and we had 1 early dislocation during follow-up. Three of the eight re-revised elbows had a third revision. Two other patients had a resection arthroplasty because of deep infection after the first revision. After 5 years, 73.8% of the revised elbow prostheses were still in situ. The lack of other satisfactory treatment options, combined with the satisfactory clinical results, justify revision surgery of elbow prostheses. The absence of intraoperative fracture during removal of the relatively small standard components and the availability of long-stemmed revision components, which facilitate fixation in the impaired bone, both support our preference for the Souter-Strathclyde prosthesis.

Proximal migration can be measured accurately on standardized anteroposterior shoulder radiographs.

UNLABELLED: During the last decade, investigators of several studies have stressed the importance of early diagnosis and treatment of rotator cuff tears to improve outcome of surgical repair. Proximal migration of the humeral head is thought to be indicative for rotator cuff disorders. We wanted to assess the accuracy of proximal migration measurement on anteroposterior radiographs. Computed tomography scans and anteroposterior radiographs of 43 shoulders in 26 patients were taken using the same protocol. Proximal migration was measured as the acromiohumeral interval and the upward migration index. The mean absolute difference for the upward migration index was only 0.06 (standard deviation, 0.07), this was less then 5% of the mean upward migration index measured on computed tomography reformations. The correlation coefficient and Bland-Altman plot showed a strong correlation between computed tomography and radiographic measurements, especially for the upward migration index (correlation coefficient, 0.82). Our results indicate that measurement of subacromial space on anteroposterior radiographs, controlled for positioning, scaling, and individual differences by using the upward migration index, presents an accurate measurement for proximal migration. LEVEL OF EVIDENCE: Diagnostic study, Level I (testing of previously developed diagnostic criteria on consecutive patients--with universally applied reference "gold" standard). See the Guidelines for Authors for a complete description of levels of evidence.

Responsiveness of the shoulder function assessment scale in patients with rheumatoid arthritis.

OBJECTIVES: To investigate the responsiveness of the Shoulder Function Assessment scale (SFA) in patients with rheumatoid arthritis (RA). METHODS: In 35 patients with RA receiving a (peri-)articular injection because of local shoulder complaints the SFA, impact of shoulder function on activities of daily living, active shoulder range of motion (ROM), the 28 joint count Disease Activity Score (DAS28), and the Health Assessment Questionnaire (HAQ) were measured before and 4-6 weeks after treatment. Responsiveness was determined using the standardised response mean (SRM), effect size (ES), and responsiveness ratio (RR). RESULTS: Overall, significant improvement was seen according to the SFA (mean change 10.9 (95% confidence interval 6.5 to 15.3)), active shoulder ROM (except external rotation), and the impact of shoulder function on daily activities. In addition, the DAS28 and HAQ scores improved significantly. The responsiveness of the SFA was excellent, with the SRM, ES, and RR being -0.86, -1.16, and 1.28, respectively. CONCLUSIONS: In addition to its good validity and reliability, the SFA proved to have a high sensitivity to clinical changes in patients with RA who received local treatment for shoulder complaints.

Pathological Teres Major activation in patients with massive rotator cuff tears alters with pain relief and/or salvage surgery transfer.

BACKGROUND: Massive rotator cuff tears impose restraints on overhand arm functionality and are often accompanied by pain. After musculotendinous Teres Major transfer, overhand arm function is generally restored and pain is reduced. The assumed mechanical abduction insufficiency and Teres Major muscle function adaptation will be experimentally verified. METHODS: Principal Teres Major muscle activation (surface IEMG averaged over 3s) is recorded for 12-24 isometric and isotonic force directions perpendicular to the 60 degrees forward flexed humerus in three conditions: prior to surgery (n = 6 patients), prior to surgery and after subacromial anaesthetic (n = 6) and post-surgery (n = 3). Principal direction and on-, offset directions were estimated. FINDINGS: Teres Major activation adapts both to pathological and post surgery conditions: the normal activation during adduction changes into activation during forward flexion or abduction. Glenohumeral stabilisation, not abduction torque, seems to be the explanation for post surgery Teres Major transfer success. INTERPRETATIONS: The pathological absence of Supraspinatus and Infraspinatus forces during forward flexion result in increased upward glenohumeral instability. The superior translations are compensated for by Teres Major activity during forward flexion. This translation-'force' function conflicts with the adduction-generating rotation-'torque' function. This may explain the pain-induced reduction of arm elevation in these patients. Musculotendinous transfer solves the force-torque conflict by changing the moment arm of the Teres Major from adduction to abduction. Teres Major can now both compensate for the loss of Supraspinatus and Infraspinatus forces needed for glenohumeral stabilisation and contribute to forward flexion of the arm.

Indications, complications, and results of shoulder arthroplasty.

OBJECTIVE: To provide estimates of patient outcomes following shoulder arthroplasty using Neer-II type humeral prosthesis and to examine variation in outcomes due to patient and prosthesis characteristics. METHODS: North American and Western European published articles were identified through a computerized literature search and bibliography review. Studies were included if they enrolled 15 or more patients, discriminated between hemi-arthroplasty (HEMI) and total shoulder arthroplasty (TSA) and measured pain relief, gain in range of motion (ROM), radiographic follow-up (> 2 years), short- and long-term complications, and revision surgery. RESULTS: A total of 40 studies satisfied the inclusion criteria. The total number of patients enrolled was 3584. The mean follow-up was 59 months. The mean patient age was 62 years, 65% of patients were women and 73% underwent TSA. All reports showed relevant pain relief, increase in ROM, and high satisfaction rates for HEMI and TSA in both osteoarthritis (OA) and rheumatoid arthritis (RA). The overall rate of revision was 8%. Significant differences between HEMI and TSA for both diagnoses were found for all outcome parameters. CONCLUSION: Shoulder arthroplasty is a safe and effective procedure for OA and RA patients. The diagnosis, shoulder pathology, and prosthesis specifics were significant predictors of outcomes. We therefore emphasize that conclusions on the outcome of shoulder arthroplasty can only be made if differentiated between these patient and prosthesis specifics. Limitations in the reporting style of these articles severely constrain the ability to explore variation in outcomes due to study, patient, or prosthesis characteristics and restrict their generalisability.

A new type of model-based Roentgen stereophotogrammetric analysis for solving the occluded marker problem.

Roentgen stereophotogrammetric analysis (RSA) measures micromotion of an orthopaedic implant with respect to its surrounding bone. A problem in RSA is that the markers are sometimes overprojected by the implant itself. This study describes the so-called Marker Configuration Model-based RSA (MCM-based RSA) that is able to measure the pose of a rigid body in situations where less than three markers could be detected in both images of an RSA radiograph. MCM-based RSA is based on fitting a Marker Configuration model (MC-model) to the projection lines from the marker projection positions in the image to their corresponding Roentgen foci. An MC-model describes the positions of markers relative to each other and is obtained using conventional RSA. We used data from 15 double examinations of a clinical study of total knee prostheses and removed projections of the three tibial component markers, simulating occlusion of markers. The migration of the tibial component with respect to the bone, which should be zero, for the double examination is a measure of the accuracy of algorithm. With the new algorithm, it is possible to estimate the pose of a rigid body of which one or two markers are occluded in one of the images of the RSA radiograph with high accuracy as long as a proper MC-model of the markers in the rigid body is available. The new algorithm makes RSA more robust for occlusion of markers. This improves the results of clinical RSA studies because the number of lost RSA follow-up moments is reduced.

Bone ingrowth simulation for a concept glenoid component design.

Glenoid component loosening is the major problem of total shoulder arthroplasty. It is possible that uncemented component may be able to achieve superior fixation relative to cemented component. One option for uncemented glenoid is to use porous tantalum backing. Bone ingrowth into the porous backing requires a degree of stability to be achieved directly post-operatively. This paper investigates the feasibility of bone ingrowth with respect to the influence of primary fixation, elastic properties of the backing and friction at the bone prosthesis interface. Finite element models of three glenoid components with different primary fixation configurations are created. Bone ingrowth into the porous backing is modelled based on the magnitude of the relative interface micromotions and mechanoregulation of the mesenchymal stem cells that migrated via the bonded part of the interface. Primary fixation had the most influence on bone ingrowth. The simulation showed that its major role was not to firmly interlock the prosthesis, but rather provide such a distribution of load, that would result in reduction of the peak interface micromotions. Should primary fixation be provided, friction has a secondary importance with respect to bone ingrowth while the influence of stiffness was counter intuitive: a less stiff backing material inhibits bone ingrowth by higher interface micromotions and stimulation of fibrous tissue formation within the backing.

Primary Souter-Strathclyde total elbow prosthesis in rheumatoid arthritis. Surgical technique.

BACKGROUND: Total elbow arthroplasty is a well-established treatment for the painful elbow joint in patients with rheumatoid arthritis. We present the results of what we believe to be the first prospective study of the Souter-Strathclyde total elbow prosthesis. METHODS: Between June 1982 and December 2000, 204 primary total elbow prostheses were inserted in 166 patients who had rheumatoid arthritis. No patient was lost to follow-up. The mean duration of follow-up was 6.4 years. All patients were examined preoperatively, at one and two years postoperatively, and at regular intervals thereafter. RESULTS: Six of the 204 elbows had pain at rest at the time of the latest follow-up. Ten patients (ten elbows) without previous neurological symptoms had development of paresthesias in the distribution of the ulnar nerve postoperatively. Patients who had pain at rest or at night and those who had ulnar nerve symptoms preoperatively were found to have a significant chance of having the same complaints postoperatively. Pain at rest or at night and a decrease in function during the follow-up period were associated with humeral loosening. Twenty-four elbows had revision of the total elbow prosthesis because of loosening of the humeral component (ten), loosening after fracture (six), dislocation (four), infection (two), restricted range of motion (one), or fracture of the middle part of the humeral shaft, proximal to the prosthesis (one). One prosthesis was removed because of humeral loosening, and eight were removed because of deep infection. Another five prostheses were radiographically loose at the time of the latest follow-up. The rate of implant survival, according to the method of Kaplan-Meier, was 77.4% after ten years and 65.2% after eighteen years. CONCLUSIONS: Total elbow replacement is associated with a high complication rate and therefore may be warranted only for seriously disabled patients. Currently, the results associated with the Souter-Strathclyde total elbow prosthesis are comparable with the results associated with other prostheses, but loosening of the humeral component remains a concern.

Treatment of recurrent posterior dislocation of the shoulder in cerebral palsy by injection with botulinum toxin A into the M. subscapularis.

SUBJECTS: A 20-year-old woman, known to have cerebral palsy and a spastic hemiparesis, suffered from frequent (up to 20 times a day) and painful posterior dislocation of the affected shoulder. INTERVENTIONS: For the last two years we have treated her with injections with botulinum toxin A (100 U Botox) in the M. subscapularis. RESULTS: Pain and dislocation rate have improved substantially.

Effect of glenoid component inclination on its fixation and humeral head subluxation in total shoulder arthroplasty.

OBJECTIVE: To investigate the effect of glenoid component inclination angle on the fixation of cemented glenoid-bone structures and humeral head subluxation. BACKGROUND: Results of shoulder replacements, in terms of glenoid component fixation and joint functionality, are bad and must be improved. Repeated glenoid component tilting, a result of eccentric contact forces harms glenoid component fixation. However, the effect of glenoid component inclination is unknown. METHODS: Keeled glenoid components are cemented into bone substitutes and positioned under inclination angles varying between -4.5 degrees and +4.5 degrees . For each inclination angle 5 glenoid specimens are loaded by a horizontal, constant joint compression force (725 (SD 10) N) and a vertical, superior directed subluxation force (shear force), cyclically varying between 0 and 350 (SD 1) N. After 200,000 load cycles, the upper and lower glenoid component rim-displacements are measured during 1500 additional load cycles by custom made displacement sensors. RESULTS: The maximal superior rim-displacement significantly increased and the minimal inferior rim-displacement dramatically decreased for increasing glenoid component inclination angles up to +4.5 degrees . Maximal ratio of subluxation force over compression force significantly decreased for increasing glenoid component inclination. CONCLUSIONS: According to this study, decreasing glenoid component inclination angle decreases glenoid component tilting and humeral head subluxation and the results can be used to improve total shoulder replacements. RELEVANCE: Better understanding of the effect of glenoid component inclination on glenoid component tilting and humeral head subluxation may be useful information for the surgeon to improve results of the total shoulder arthroplasty, when replacing the glenoid surface.

Effect of joint conformity on glenoid component fixation in total shoulder arthroplasty.

Results of shoulder replacements are inferior and must be improved. Two of the major problems of total shoulder replacements are loosening of cemented glenoid components and wear of polyethylene inlays of uncemented, metal-backed glenoid components. The aim of this study is to investigate the influence of joint conformity on glenoid-component fixation. Keeled glenoid components, with radii of curvature of 24, 25, or 29 mm, were cemented in bone substitutes, placed in a force-controlled test set-up, articulating against a 24 mm humeral head. They were loaded by a constant joint compression force (725 +/- 10 N) and a superior subluxation force (shear force), cyclically varying between 0 and 350 +/- 1 N. After 200,000 load cycles, the upper and lower glenoid component rim-displacements were measured by custom-made displacement sensors. Additionally, the shear-out strength has been measured to investigate the residual strength. The glenoid component structures with radii of curvature of 24, 25, and 29 mm showed maximum superior rim-displacements of 0.163 (SD = 0.01), 0.299 (SD = 0.0306), and 0.350 (SD = 0.0197) mm respectively, which is a significant difference (p < 0.05). The maximum shear-out strength of glenoid components with radii of curvature of 24, 25, and 29 mm was 2707 (SD = 452), 2648 (SD = 299), and 2631 (SD = 312) N respectively, which is not a significant difference (p < 0.05). However, the results indicate that a conform articulation shows smaller glenoid rim-displacements, which might be beneficial for long-term component fixation.

Evaluation of three pose estimation algorithms for model-based roentgen stereophotogrammetric analysis.

Model-based roentgen stereophotogrammetric analysis (RSA) uses a three-dimensional surface model of an implant in order to estimate accurately the pose of that implant from a stereo pair of roentgen images. The technique is based on minimization of the difference between the actually projected contour of an implant and the virtually projected contour of a model of that same implant. The advantage of model-based RSA over conventional marker-based RSA is that it is not necessary to attach markers to the implant. In this paper, three pose estimation algorithms for model-based RSA are evaluated. The algorithms were assessed on the basis of their sensitivities to noise in the actual contour, to the amount of drop-outs in the actual contour, to the number of points in the actual contour and to shrinkage or expansion of the actual contour. The algorithms that were studied are the iterative inverse perspective matching (IIPM) algorithm, an algorithm based on minimization of the difference (DIF) between the actual contour and the virtual contour, and an algorithm based on minimization of the non-overlapping area (NOA) between the actual and virtual contour. The results of the simulation and phantom experiments show that the NOA algorithm does not fulfil the high accuracy that is necessary for model-based RSA. The IIPM and DIF algorithms are robust to the different distortions, making model-based RSA a possible replacement for marker-based RSA.