33

• Achieving acetabular implangt stability after svere bone lossand pelvic discontinuity. Lessons from a case report.

John Abrahams^1,2; Stuart Callary^1,2; Robyn Clothier^1,2; Donald Howie^1,2; Lucian B Solomon^1,2

¹Department of Orthopaedics and Trauma, Royal Adelaide Hospital, Adelaide, South Australia

²Centre for Orthopaedic and Trauma Research, The University of Adelaide, Adelaide, South Australia

Introduction: The acetabular components used in revision total hip arthroplasty (THA) to treat severe acetabular bone defects have high rates of re-revision at mid to long-term follow-up. A scoping review demonstrated that RSA studies of acetabular revisions treating larger bone defects or pelvic discontinuity report larger amounts of implant migration (Abrahams et al, JBJS-Rev 2020;8(4):e0170). A case report of a 72 year old female is presented that underwent revision THA for acetabular loosening and osteolysis.

Material and Methods: Reconstruction of the patient’s Paprosky III acetabular defect with pelvic discontinuity was undertaken initially by using a double cup technique. Tantalum beads were placed in the ilium and ischium to determine the migration between the acetabular component and the ilium and ischium separately. Migration of the ischium with reference to the ilium was also determined.

Results: The initial reconstruction led to osteointegration of the construct to the ilium but not the ischium. Acetabular component proximal migration was less than 1.0mm with respect to the ilium. The migration was 5.5mm when using the ischium tantalum markers as a reference. During re-operation for recurrent dislocation, the acetabular component was confirmed to be well fixed to the ilium but not the ischium. The lack of osteointegration of the construct to the ischium was thought to be related to the insufficient contact between the inferior cup and the ischium/pubis. Migration across the pelvic discontinuity was present until the acetabular component was re-fixed to the ischium with screws after re-reaming to bleeding host bone, to allow for ingrowth. Re-stabilization of the construct to the ischium failed until the inferior cup was revised and good contact of the implant to bone was achieved.

Interpretation and Conclusion:This case underlines the importance of good initial contact and stability in achieving acetabular component integration even in the presence of minimal bone to implant contact.

Keywords: Revision THA; Acetabular defects, Discontinuity; Cup Migration

34

• Migration of the BioPro MTP-1 hemiarthroplasty analysed with Roentgen Stereophotogrammetric Analysis: A Pilot Study

V.V. Balesar¹ MD, Lennard A. Koster² MSc., Bart L. Kaptein² PhD, S.B. Keizer¹ PhD MD

¹ Department of Orthopaedic Surgery, Haaglanden Medical Center, The Hague, the Netherlands.

² Biomechanics and Imaging Group, Department of Orthopaedic Surgery, Leiden University Medical Center, the Netherlands.

Introduction: Mixed results for functional outcome and long-term fixation are reported for end-stage hallux rigidus joint replacement arthroplasty [1, 2]. This Roentgen Stereophotogrammetric Analysis (RSA) cohort study was designed to evaluate the feasibility of measuring micromotion of the BioPro metatarsophalangeal-1 joint prosthesis with Model-based RSA. Migration patterns of the prosthesis and prosthesis-induced erosion of the metatarsal bone were evaluated up to five years post-operative (PO).

Material and method: Eleven consecutive patients with end-stage hallux rigidus received the BioPro-1 hemiprosthesis. RSA markers were inserted around the prosthesis for RSA 3-dimensional translation, rotation and erosion measurements at direct-PO, 6-weeks and 3, 6, 12, 36 and 60 months post-operatively.

Results: RSA data of nine patients was available for analysis. Median (range) number of markers used in RSA analysis, condition number and mean error of markers around the prosthesis were
4 (3 – 7), 300 (208 – 833) and 0.13 (0.02 – 0.28) respectively. A linear mixed model for subsidence of the prosthesis showed progressive subsidence up to 3 years PO (mean 2.1 mm, SD 1.3), after which subsidence ceased. Although, two individual prostheses continued to migrate. Progressive metatarsal erosion was found up to 1 year PO (mean 0.5 mm. SD 0.5) and showed stabilisation afterwards.

Interpretation and conclusion: Marker distribution in the proximal phalanx is difficult and does not allow for accurate rotation measurements. Model-based RSA is possible for translations of the BioPro-1 prosthesis and shows subsidence up to 3 years PO and metatarsal erosion up to 1 year PO.

Keywords: Radiostereometry, MTP-1 joint replacement, Hallux rigidus

35

• In vivo biomechanics assessment of a CR total knee prosthesis during sit to stand: coupling dynamic RSA and FE analysis

Agostino Igor Mirulla¹, Laura Bragonzoni², Raffaele Zinno², Stefano Zaffagnini^1,3, Bernardo Innocenti⁴

Affiliations
1. Department of Biomedical and Neuromotor Sciences, University of Bologna, Italy;

2. Department for Life Quality Studies, University of Bologna, Italy;

3. 2nd Orthopaedic and Traumatologic Clinic, IRCCS Istituto Ortopedico Rizzoli, Italy;

4. BEAMS Department (Bio Electro and Mechanical Systems), Université Libre de Bruxelles, Belgium

Introduction

Total knee arthroplasty (TKA) is the gold standard treatment for patients with primary osteoporosis, as it can relief pain and restore joint function. However, there is stillroom for improvement as far as prosthesis behavior are concerned.

In this study, an innovative technique, combining RSA dynamic and patient-specific finite element models, was applied to analyze a group of 15 patients who underwent total knee arthroplasty with a cemented PCL highly congruent MB TKA during the execution of a sit to stand from chair.

The aim of our study was to evaluate the position and translation of the femoral polyethylene insert contact points.

Materials and Methods

A cohort of 15 patients, underwent cemented CR highly congruent MB TKA (Gemini, Waldemar LINK GmbH & Co. KG, Barkhausenweg 10, 22339 Hamburg, Germany) with patella resurfacing for primary OA, was evaluated at minimum 9-month follow-up using Model-based dynamic RSA in weight-bearing conditions and during the execution of a sit to stand from chair. The in vivo kinematic data obtained from RSA Dynamic was used as an input for a patient specific finite element models of the implants to calculate condylar contact points between the femoral component and polyethylene insert. The contact points were determined by the FE software as the centroid of the pressure distribution between the femoral condyles and the polyethylene insert.

Results

The AP translation showed that the femoral component, during the sit to stand, performed an anterior translation during the last 40° of flexion (range 4 mm). Concerning the IE rotation, femoral component performed an internal rotation (range 8.18°) during the whole task.

Interpretation and Conclusions

This study evaluated the knee kinematics in terms of articular surface contacts. The anterior translation showed from 40° to 0° may represents a paradoxical anterior translation.

Keywords: FEM, RSA, total knee arthroplasty, contact points.

36

• Methodology for Tracking Scoliosis Spine Fusion in a Standing Low-Dose Biplanar X-ray Imager using RSA

Jennifer Hurry, Alan Spurway, Benjamin Orlik and Ron El-Hawary

IWK Health Centre, Halifax, Nova Scotia, Canada

Introduction: Our aim is to assess time to fusion for PSIF scoliosis surgery in adolescent patients by measuring intervertebral motion. This pilot study also examined the protocol needed to capture reliable RSA images in the EOS imager.

Material and method: The first three participants (mean 14.2 years old) had 1mm tantalum beads implanted during surgery (five or six vertebrae marked, maximum of two vertebrae unmarked between bead clusters), and were imaged in the biplanar imager at six weeks, three, six, and approximately twelve months. After the six week exam, traction and compression to patient tolerance were applied during imaging to assess the inducible displacement of the vertebrae. Imager settings were adjusted for RSA scans, with scan speed and voltage increased, and the EOS module for MBRSA was used to analyze the images.

Results: The mean intervertebral motion between two standing exams at 6 weeks was 0.02, 0.07, and 0.11mm (x,y,z). There was no visible trend in the induced displacements during the follow up exams with average translation components between -0.29 and 0.11 mm (max standard deviation of 0.54 mm) and average rotation components between -0.08 and 0.43° (max standard deviation 0.68°). When taken as a total translation vector magnitude, the average intervertebral motion measures 0.24 to 0.88 mm and adding all the intervertebral x, y, z components to calculate a total translation magnitude over the instrumented spine (12-15 levels) resulted in a total induced motion of 0.66 to 3.28 mm, and a maximum of 0.75 mm vertically. Any patient sway during imaging is included in the results.

Interpretation and conclusion: Early measurements suggest that patients can be stabilized during the scanning, and there is little motion between vertebrae after 3 months. Further sway removal may be needed to distinguish a fusion trend after this time.

Keywords: spine, scoliosis, EOS imager

37

• Stable polyethylene fixation and low polyethylene wear-rate in fixed-bearing total knee arthroplasty at 5-6 years follow-up

Jonathan Hugo Jürgens-Lahnstein^b, Emil Toft Petersen^b,c, Søren Rytter^a, Frank Madsen^a, Kjeld Søballe ^a,b, Maiken Stilling^a,b,c. ^aDepartment of Orthopedics, Aarhus University Hospital, Denmark. ^bDepartment of Clinical Medicine, Aarhus University, Denmark. ^cUniversity Clinic for Hand, Hip and Knee Surgery, Holstebro Regional Hospital, Denmark

Background: Micromotion of the polyethylene (PE) inlay may contribute to back-side PE wear in addition to articulate wear of total knee arthroplasty (TKA). The stabilizing effect of the locking mechanism for the PE inlay is important. Using radiostereometric analysis (RSA) with tantalum beads in the PE inlay we evaluated PE micromotion and its relationship to PE wear.

Patients and methods: 23 patients (10 male) mean age 83 years were available from an RSA study on cemented TKA with Maxim tibial components. PE inlay migration, wear, tibial component migration and the anatomical knee axis were evaluated on weight-bearing stereoradiographs. CAD surface models of the implant components were used to measure the PE inlay wear as the deepest penetration of the distal femoral component surface into the tibial PE inlay.

Results: At mean 6 years follow-up, the PE wear rate was mean 0.08 mm/year (95% CI 0.07 – 0.10) in the lateral compartment and 0.07 mm/year (95% CI 0.05 – 0.09) in the medial compartment. The PE-inlay external rotation in relation to the tibial component was mean -0.12° (95% CI: -0.55 – 0.31), which was below the precision limit. Tibial components were well-fixed, but varus knee alignment increased tibial component total translation (rho 0.72, p=0.04). 

Conclusion: PE micromotion was below the measure limit with radiostereometry and PE wear was similar for the medial and lateral compartment. We found no relationship between PE stability and PE wear. Alignment of the anatomical axis did not influence PE wear.

Keywords: Knee arthroplasty; Cemented; Radiostereometry; Polyethylene; Wear; locking-mechanism

38

• Migration pattern of cemented Exeter Short Stem in Dorr type A femurs – A prospective radiostereometry study with 2-year follow-up

Dahl Vind T^1,2, Jørgensen PB, Vainorius D, Jakobsen SS, Søballe K, Stilling M

¹ AutoRSA Research Group, Department of Orthopaedics, Aarhus University Hospital, Aarhus, Denmark

² Department of Clinical Medicine, Aarhus University, Aarhus, Denmark

³ University Clinic for Hand, Hip, and Knee Surgery, Holstebro, Denmark

Introduction: The Exeter short stem (ESS) is 25 mm shorter than the standard length v40 Exeter stem (Stryker) and intended for use in a narrow femoral diaphysis. The purpose of the study was to evaluate the migration pattern of the cemented ESS.

Material and Method: In a prospective single-center cohort study, 23 patients (21 female) mean age 78 (range 70-89) with hip osteoarthritis and Dorr Type A femurs were included. Preoperative DXA was used to group patients into normal (> -1) and low (<-1) T-score. Components were the collarless polished double-tapered Exeter short stem type N°1 L125. Patients were followed for two years with model-based RSA (stem migration), regular hip radiographs (stem position and cementation quality), Oxford Hip Score (OHS) and VAS pain.

Results: At two-years follow-up, the stems subsided 1.48 mm (CI95 1.69; 1.26) and retroverted 0.45° (CI95 0.01; 0.88). From 12 to 24 months, stem subsidence was 0.18 mm (CI95 0.1; 0.25) (p=0.001) and retroversion was -0.04° (CI95 -0.27; 0.18) (p=0.70). T-score and stem subsidence correlated (rho=0.48; p=0.025) and patients with normal T-score (n=7) had 0.42 mm (CI95 -0.01; 0.85) less subsidence as compared to patients with low T-score (n=15) (p=0.054). Stems in varus position (n=10) subsided 1.7 mm (CI95% 1.35; 2.05) compared to 1.33 mm (CI95% 1.05; 1.60) for stems in neutral position (n=13) (p=0.07). Postoperative cementation quality did not influence stem migration. OHS improved to 40.7 (CI95 36.8; 44.7) and VAS pain at rest and activity decreased to 5mm and 10mm, respectively (p<0.001).

Interpretation and conclusion: The migration pattern of the cemented ESS was similar to reports for the cemented standard length Exeter stem. Low preoperative T-score and varus stem-position showed a tendency for higher stem migration and should be studied as risk factors for failure in larger studies of cemented polished stems.  

Keywords: Hip arthroplasty, stem migration, bone cement, radiostereometry, T-score