45

• A three-dimensional CT technique to assess early implant migration and radiolucent lines in total shoulder arthroplasty.

Cyrus Brodén¹, Peter Reilly², Monica Khanna³, Ravi Popat², Olof Sköldenberg⁴, Henrik Olivecrona⁵, Roger Emery¹.

¹Department of Surgery and Cancer, Imperial College London, St Mary’s Hospital,W2 1NY, London, UK,

²Department of Bioengineering, Imperial College London, Exhibition Road, SW7 2AZ, London, UK.

³Department of Clinical Imaging, Imperial College Healthcare NHS Trust, Praed Street, London, W2 1NY, UK.

⁴ Karolinska Institutet, Department of Clinical Sciences, Danderyd Hospital, Division of Orthopaedics, SE-182 88 Stockholm, Sweden.

⁵Department of Molecular Medicine and Surgery, Karolinska Institutet, 17176 Stockholm, Sweden

Introduction: Computed micromotion analysis (CTMA) is a tool relying on CT images to measure early migration of orthopaedic implants. Early migration and development of radiolucent lines are believed to be correlated to loosening of implants. The aim of this study is to investigate the feasibility of CTMA to assess early migration and the progression of radiolucent lines over a period of 2 years in patients with shoulder arthroplasty using sequential CT scans.

Material and Method: Seven patients were included in this study and underwent nine primary shoulder arthroplasties. CT scans were conducted preoperatively, immediately postoperatively and after 3,6,12 and 24 months. At each follow-up, postoperative glenoid and humeral component migration, and the development of radiolucent lines were assessed in CTMA. Clinical scores were recorded at all time points except immediately postoperatively.

Results: For the glenoid component, the median translation and the median rotation was in the range of 0.00–0.10 mm and -1.53–1.05° at 24 months. The progression of radiolucent lines occurred from the periphery to involve the pegs of the glenoid components during the 24-month follow-up after surgery. For the humeral component, the median translation and the median rotation varied between -0.02 and -0.01 mm, and between -0.62 and -0.06° at 24 months. The Constant Score improved from a mean of 30 (21–51) preoperatively to 70 (41–88) at 24 months, while the Oxford Score improved from a mean of 24 (13–38) preoperatively to a mean of 43 (32–48) after 24 months.

Interpretation and conclusion: CTMA can be used to identify early migration and the development of radiolucent lines over time in shoulder arthroplasty. Clinical trials with a larger sample size and longer follow-up are needed to establish the relationship between migration, radiolucent lines and clinical scores.

Keywords: CT, RSA, hip arthroplasty, CTMA, migration

46

• Precision measurements of CTMA and RSA methods. Can the former replace the latter?

 Angelomenos Vasileios, Mohaddes Ardebili Maziar, Itayem Raéd, Shareghi Bita

Introduction: Marker-based Radiostereometric Analysis (RSA) is the most accurate clinical method for determining early micromotions of orthopaedic implants. Computed Tomography Micromotion Analysis (CTMA), developed by Sectra in Sweden, is an analysing tool that can be used to determine implant micro-movements using low-dose CT scans. The main advantage of this method is that markers do not need to be attached in the skeleton or to the implants as traditional marker-based RSA requires. By using CTMA, a non-invasive measurement of joint movement is enabled. The purpose of this study was to evaluate the precision of measurements performed with CTMA by using marker-based RSA as reference. We hypothesised that CTMA can be used as an alternative to RSA in assessing implant micromotions.

Materials and methods: We included 20 patients initially part of a larger ongoing study at Sahlgrenska university hospital, Gothenburg, Sweden. To determine the precision of both methods, double examinations (postoperatively) with repositioning of the patients were performed. The precision was calculated from zero by assuming that there was no motion of the prosthesis between the two examinations.

Results: We found no significant differences between the methods regarding acetabular cup migration and rotation, p-value > 0.05. This suggests that the precision of the two methods is comparable. However, CTMA displayed an insignificant higher precision in measuring cup rotations.

Interpretation and conclusion: Precision of the CTMA method according to our data is as trustworthy as that of RSA. Our data indicate that the precision of CTMA in measuring acetabular cup migration and rotation is comparable to the classical marker-based RSA and can be used as an alternative to the standard method.

Keywords: RSA, CTMA, Precision, Migration

47

• Inducible displacements of the Sacroiliac joint measured both with RSA and CTMA – A pilot study

Vinjar Brenna Hansen¹, Olof Sandberg², Stephan M. Röhrl³

*Correspondence: vinjar.brenna.hansen@helse-bergen.no

¹Departement of Orthopedic Surgery, Haukeland University Hospital, Bergen, Norway

² Sectra AB, Sweden

³Departement of Orthopedic Surgery, Oslo University Hospital, Ullevål, Norway

Background: Movement of the sacroiliac joint (SI joint) is difficult to measure. RSA has been the golden standard but requires operative insertion of markers into the joint. CT Motion Analysis (CTMA) is a CT based measuring method for inducible displacements using similar algorithms as RSA. CTMA has earlier been documented in the spine but has never been used to evaluate movement of the SI joint. We want to compare the feasibility of measurements of inducible displacements comparing RSA and CTMA.

Method/design: We recruited 2 patients (age 45 and 59) from an earlier study conducted at Ullevål University Hospital in Norway. Both patients had chronic pelvic girdle pain and were fused surgically in 2009. Tantalum markers were inserted into both ileum, symphysis and the sacrum surgically before a second stabilizing operation of the pelvic ring 2 months later. We performed 2 different inducible displacements. First, an anterior straight leg raise (ASLR) and second, a figure-of-four position, both with 1kg of load and on both sides. Movements were presented numerically, by pictures with a color code and overlay picture animations.

Results/interpretation: In one patient the joint still moved in spite of the fusion.  During ASLR provocation higher degree of movement was measurable in the contralateral SI joint. Movements were smaller during the figure of 4 provocations and mainly detectable in the ipsilateral ileum. Motion could be visualized during the analysis process with CTMA.

Conclusion: Both methods were able to detect movements in the SI joint. A direct comparison is not possible since the data was not collected during the same provocation. Detection of markers was facilitated by the 3- dimensional CT images. CTMA technique provides numerical and visual interpretation of movements. Our pilot study encourages further research with CTMA for SI joint pathology.

Keywords: RSA, CT method, CTMA, low dose CT, IS joint movement, pelvic girdle pain

48

• Computer tomography (CT) based micromotion analysis of the Lisfranc joint: A pilot study

Magnus Poulsen MD , Are H. Stødle MD, Stephan M. Röhrl MD PhD. Division of Orthopaedic Surgery, Oslo University Hospital, Oslo, Norway.

Objective: Accurate midfoot range of motion is difficult to quantify by radiographic imaging alone. In this pilot study we explore the application of a new CT-based micromotion analysis (CTMA) software combined with Cone beam–CT (CBCT) examinations of the Lisfranc joint under physiological load. Here we examine and compare 1st tarsometatarsal (TMT)-joint range of motion in patients previously treated for a Lisfranc injury with a temporary bridge plate fixation.

Method: We examined 6 feet (3 patients). All patients had previously been treated with a unilateral, temporary bridge plate fixation over the 1st TMT joint. Minimum follow-up time was five years post-operative before inclusion. We obtained CBCT examinations during non- and full weight-bearing sequences. CT-protocol consisted of low-dose (0.2mSv), 0.6mm slice scans with metal artifact reduction. 1st TMT joint motion was analyzed using CTMA (SECTRA, Sweden) with the medial cuneiform as a fixed object and the 1st metatarsal as a moving object. Motion of interest, dorsiflexion (DF) and sagittal translation (ST), were documented and compared with the contralateral side. Precision was determined by twelve double examinations where we registered the proximal and distal part of 1st metatarsal separately and calculated the motion between these parts. Final result was determined as 1.96xSD.

Results: The image quality and resolution of our CBCT examinations worked well with the CTMA software. For measurement precision the highest were 0.5° and 0.2mm for x, y, and z. Overall, the 1st TMT motion was very similar between the two examined feet. On the nonsurgical side, median DF were 1.7° (2.5, 1.7, 1.4) compared to 1.5° (2.6, 1.5, 0.4) on the operated foot. Furthermore, median ST were 1.2mm (1.0, 2.0, 1.2) compared to 1.0mm (1.0, 1.1, 0.2) between the nonsurgical and surgical foot respectively.

Conclusion: The use of CTMA together with CBCT is appliable to micromotion analysis of the Lisfranc joint with comparable precision as previously published studies on other joints. Regaining natural Lisfranc motion after treatment with a temporarily bridging plate is observable in our small patient group. This pilot study encourages further evaluation of CTMA as a tool to measure midfoot motion.

49

• Initial experiences with CT-IMA in selected cases of the hip and knee

Mathilde Kvamme, MD., Stephan M. Röhrl, MD PhD. Orthopaedic department, Oslo University Hospital.
Center for Implant and Radiostereometric Research Oslo

Introduction: Diagnostics of definitive implant loosening is a challenge in some patients. This has a high impact on treatment strategy, which might have considerable consequences for the patient. Often conventional radiology and static CT does not give a conclusive answer to decide whether or not a revision is indicated. Recently CT-based Image Motion Analysis (IMA) has been introduced as a new and promising method for assessing implant migration. In this pilot study we wanted to examine whether the new method might help to diagnose implant loosening or to detect knee ligament instability.

Material and methods: We included 5 patients, 3 with THA and 2 patients with TKA. In all patients we investigated implant loosening (1 primary, 1 long revision stem, a ceramic cup and 2 TKA) and in one patient also ligament instability after TKA. The latter had tantalum beads inserted and we performed RSA stress radiographs. IMA was compared to conventional radiography and simultaneously CT-scans.

Results: CT suggested “possible loosening” for two patients, and IMA “confirmed loosening” with clear visualization of the implant movement. Implant stability was confirmed for 2 TKA and a ceramic cup by IMA. Knee instability could be quantified with IMA and RSA. 

Conclusion: In selected cases IMA adds information to the diagnostics of implant loosening and knee instability. It offers more quantifiable answers and visual interpretation to base a clinical decision. But this needs to be verified in larger patient series and for more implants.

Key words: IMA, RSA, implant loosening, arthroplasty, clinical, pilot study

50

• Implant migration and bone mineral density changes can be measured simultaneously with low-dose CT scans – a prospective study on 17 acetabular revisions with impaction bone grafting.

H. Stigbrand*, K. Brown**, H. Olivecrona*** and G. Ullmark*. *Dept. of Orthopedic Surgery, Gävle Hospital, Sweden ** Mindways Software Inc, Texas, USA, *** Dept of Orthopedic Surgery, Karolinska Institute, Sweden.

Introduction: Serial low-dose CT-scans post-op makes implant migration and bone mineral density (BMD) analysis possible. In this prospective study we investigated cup migration and BMD changes after acetabular reconstruction in revision total hip arthroplasty with impaction bone grafting (IBG).

Material and Method: 17 patients had a cup revision using IBG and a graft compressing titanium shell. A cup was cemented inside the shell. Six patients had combined segmental and cavitary acetabular bone defects. Low-dose CT-scans were performed post-op, after 6 weeks and 2 years.

Cup migration was analyzed by CT-based implant motion analysis with a novel software. No tantalum markers in the host bone were used.

Bone mineral density was analyzed in the bone graft and in native adjacent bone by using volumetric Quantitative Computerized Tomography.

Results: At two years translations of the cup were 1.52 (95% CI 0.4-2.6) mm in proximal direction, -0.56 (CI 1.6 – 0.4) mm in medial direction and 0,27 (CI 0.0-0.6) mm in anterior direction. The accuracy of the migration analysis was determined by 12 double examinations to 0,11-0,14 mm translation.

Graft BMD increased mean 14% after 6 weeks, and 23% after 2 years.

There was one mechanical failure. That patient had a pelvic dissociation, the greatest cup migration, and the second largest bone grafted volume.

Interpretation and Conclusion:

1. Low-dose CT-scans can measure implant migration and BMD in the same setting.
2. There was a rapid increase of BMD in the bone graft – a sign of rapid bone formation within the bone graft.
3. Implant migration can be measured with high accuracy using low dose CT-scans without tantalum markers.
4. Proximal migration of the acetabular component was comparable to previous reports.