Jacob T. Munro

Pre-formed articulating knee spacer in two-stage revision for the infected total knee arthroplasty

We performed a prospective study to assess safety and effectiveness of a pre-formed articulating spacer made of gentamicin-impregnated acrylic cement in the management of infected total knee arthroplasty. Twenty-one consecutive patients with unilateral deep infection were treated by two-stage revision in two centres. Two patients were excluded, and 19 patients remained available for assessment. The mean implantation time of the spacer was 12 weeks. The rehabilitation programme between stages consisted in early range of motion exercises and partial weight bearing. Mean follow-up after removal of the spacer and insertion of the final prosthesis was 24 (range, 12–43) months. No patient had recurrence of infection at the latest follow-up. The mean Knee Society functional score during spacer management was rated 75 points and was rated 84 points at the latest follow-up. No device-related complication was observed.RésuméNous avons étudié prospectivement la sécurité et l’efficacité d’un espaceur articulé pré-formé fait en ciment acrylique imprégné de gentamicine dans la gestion de l’arthroplastie totale infectée du genou. Vingt et un malades consécutifs avec une infection profonde monolatérale ont été traités dans deux centres, avec une révision en deux temps. Deux malades ont été exclus, et 19 malades sont restés disponibles pour l’estimation. Le temps d’implantation moyen de l’espaceur était de 12 semaines. La rééducation entre les deux étapes a consisté en un travail précoce de la mobilité et un appui partiel. Le suivi moyen après réimplantation prothétique était de 24 mois (12 à 43 mois). Aucun malade n’avait de récidive de l’infection au dernier recul. Le score fonctionnel moyen (Knee Society) pendant le port de l’espaceur a été estimé à 75 points et à 84 points au dernier recul. Aucune complication liée à l’espaceur n’a été observée.

Periprosthetic femoral fractures after total hip arthroplasty.

Background:  The management of periprosthetic fracture following a total hip arthroplasty is difficult, requiring expertise in both trauma and revision surgery. With rising numbers of patients in the population living with hip prostheses in situ, the frequency of these fractures is increasing, and controversy remains over their ideal management. The objective of this study was to review all periprosthetic fractures at a single institution to identify injury and treatment patterns and their associated clinical outcomes.

Quantitative computer-assisted osteodensitometry in total hip arthroplasty

Several factors can cause bone loss and fixation failure following total hip arthroplasty (THA), including polyethylene wear debris, implant micromotion and stress shielding. Various techniques have been used in an effort to detect bone density loss in vivo, all with varying success. Quantitative computed tomography (qCT)-assisted osteodensitometry has been shown to be useful in assessing the in vivo structural bone changes after THA. It has a high resolution, accuracy and reproducibility, thereby making it a useful tool for research purposes, and it is able to differentiate between cortical and cancellous bone structures and assess the bone/implant interface. This technique also provides valuable information about the pattern of stress shielding which occurs around the prosthesis and can show early bony changes, which may prove informative about the quality of implant fixation and surrounding bone adaptation. In conjunction with finite-element analysis, qCT is able to generate accurate patient-specific meshes on which to model implants and their effect on bone remodelling. This technology can be useful to predict bone remodelling and the quality of implant fixation using prostheses with different design and/or biomaterials. In the future, this tool could be used for pre-clinical validation of new implants before their introduction in the market-place.RésuméPlusieurs facteurs peuvent causer une perte osseuse et la faillite de la fixation après une arthroplastie totale de la hanche. Ils incluent les débris de polyéthylène, la micromobilité des implants et le transfert de contraintes. Plusieurs techniques ont été utilisées pour détecter la perte de densité osseuse, avec des succés variés. L’ostéodensitométrie quantitative par scanner s’est montrée utile dans l’étude in vivo des modifications structurales osseuses après arthroplastie totale de la hanche. Elle a une haute résolution, une précision et une reproductibilité qui en font un outil approprié pour la recherche. L’ostéodensitométrie quantitative peut différencier l’os cortical et l’os spongieux, étudier l’interface os-implant et donner des informations sur le modèle de déviation des contraintes qui surviennent autour d’une prothèse. Elle peut montrer précocement des modifications osseuses, ce qui renseigne sur la qualité de la fixation des implants et l’adaptation de l’os voisin. En conjonction avec l’analyse par éléments finis elle peut générer un maillage précis spécifique du patient permettant l’étude de modèles d’implants et leur effet sur le remodelage osseux. Cette technologie peut être utile pour prévoir le remodelage osseux et la qualité de la fixation pour des prothèses de différentes formes et/ou matériaux. Dans le future cet outil pourra être utilisé pour la validation pré-clinique de nouveaux implants avant leur introduction sur le marché.

Bone remodelling in the natural acetabulum is influenced by muscle force‐induced bone stress

A modelling framework using the international Physiome Project is presented for evaluating the role of muscles on acetabular stress patterns in the natural hip. The novel developments include the following: (i) an efficient method for model generation with validation; (ii) the inclusion of electromyography-estimated muscle forces from gait; and (iii) the role that muscles play in the hip stress pattern. The 3D finite element hip model includes anatomically based muscle area attachments, material properties derived from Hounsfield units and validation against an Instron compression test. The primary outcome from this study is that hip loading applied as anatomically accurate muscle forces redistributes the stress pattern and reduces peak stress throughout the pelvis and within the acetabulum compared with applying the same net hip force without muscles through the femur. Muscle forces also increased stress where large muscles have small insertion sites. This has implications for the hip where bone stress and strain are key excitation variables used to initiate bone remodelling based on the strain-based bone remodelling theory. Inclusion of muscle forces reduces the predicted sites and degree of remodelling. The secondary outcome is that the key muscles that influenced remodelling in the acetabulum were the rectus femoris, adductor magnus and iliacus.

Enhanced Recovery After Surgery in elective hip and knee arthroplasty reduces length of hospital stay.

Standardized perioperative care within an Enhanced Recovery After Surgery (ERAS) programme aims to reduce postoperative morbidity and length of hospital stay (LOS). This study evaluated the effect of ERAS in patients undergoing elective, primary total hip and knee arthroplasty (THA and TKA) in a New Zealand public hospital.

Quantitative CT-assisted osteodensitometry of femoral adaptive bone remodelling after uncemented total hip arthroplasty

The aim of this prospective study was to measure bone density changes and to assess adaptive bone remodelling after uncemented total hip arthroplasty with a taper-design femoral component using quantitative computer-tomography-assisted osteodensitometry. This method is able to differentiate cortical and cancellous bone structures. Twenty-seven consecutive patients (29 hips) with degenerative joint disease were enrolled in the study. Serial clinical, radiological and CT-osteodensitometry assessments were performed after the index operation. At the 2-year follow-up, the clinical outcome was rated satisfactory in all hips. The radiological assessment showed signs of osteointegration and stable fixation of all cups and stems. We observed a −17% decrease of cortical bone density and −22% decrease of cancellous bone density in the greater trochanter and femoral neck region. Cortical and cancellous bone density decrease at the level of the lesser trochanter was −9% and respectively −4%. We observed small changes of cortical bone density in the diaphyseal regions; in contrast, cancellous bone density increased (range 6% to 27%) in the diaphyseal regions. Overall, a trend of bone density recovery was observed throughout the follow-up period. Periprosthetic bone density changes at the 2-year follow-up are suggestive of stable osteointegration with proximal femoral diaphysis load transfer and moderate metaphyseal stress-shielding.RésuméLe but de cette étude prospective est de mesurer par scanner et ostéodensitométrie la densité osseuse et le remodelage osseux après prothèse non cimentée. Cette méthode permet de différencier les structures osseuses corticales et spongieuses. 27 patients consécutifs (29 hanches) présentant une coxarthrose ont été inclus dans cette étude qui a comporté une analyse radiologique, scanographique et ostéodensitométrie. Pour toutes les hanches à deux ans de suivi post-opératoire, le devenir clinique de ces patients était satisfaisant. La radiologie montrait des signes d’ostéointégration et de fixation stables dans toutes les cupules et pour toutes les pièces fémorales. Nous avons observé une diminution de 17% de la densité corticale et de 22% de la densité de l’os spongieux au niveau du grand trochanter et au niveau du calcar. Il existe également une diminution de la densité osseuse au niveau du petit trochanter tant sur le plan cortical 9% qu’au niveau de l’os spongieux 4%. Nous avons également observé de petites modifications de la densité osseuse corticale au niveau de la région diaphysaire, a contrario nous avons également mis en évidence une augmentation de la densité osseuse de l’os spongieux à ce niveau (de 6 à 27%). Ces différentes modifications osseuses nous permettent, à deux ans de suivi post-opératoire, de penser que l’ostéointégration proximale des éléments prothétiques entraîne un transfert de charge et un stress-shielding métaphysaire modéré.

Validation of an efficient method of assigning material properties in finite element analysis of pelvic bone.

Bone in the pelvis is a composite material with a complex anatomical structure that is difficult to model computationally. Rather than assigning material properties to increasingly smaller elements to capture detail in three-dimensional finite element (FE) models, properties can be assigned to Gauss points within larger elements. As part of a validation process, we compared experimental and analytical results from a composite beam under four-point load to FE models with material properties assigned to refined elements and Gauss points within larger elements. Both FE models accurately predicted deformation and the analytical predictions of internal shear stress.

Finite element analysis of retroacetabular osteolytic defects following total hip replacement

Periprosthetic osteolysis in the retroacetabular region with cancellous bone loss is a recognized phenomenon in the long-term follow-up of total hip replacement. The effects on load transfer in the presence of defects are less well known. A finite element model incorporating a retroacetabular defect behind a cementless component was validated against a 4th generation sawbone pelvis. Computational predictions of surface strain and von Mises stresses were closely correlated to experimental findings. The presence of a cancellous defect increased von Mises stress in the cortical bone of the medial wall of the pelvis. At a load of 600 N this was under the predicted failure stress for cortical bone. Increases in the cup size relative to the acetabulum caused increased stress in the cortical bone of the lateral wall of the pelvis, adjacent to the acetabulum. We are confident that our modeling approach can be applied to patient specific defects to predict pelvis stress with large loads and a range of activities.

Injectable thermosensitive gelling delivery system for the sustained release of lidocaine

BACKGROUND Patients undergoing arthroplasty require appropriate postsurgical pain relief. Analgesia is typically achieved through bolus doses of short-acting local anesthetics and with oral analgesics such as opiates, which are associated with systemic side effects. By formulating an injectable thermosensitive gelling system containing lidocaine, sustained and local delivery can be achieved following a single administration. RESULTS Poloxamer-based thermosensitive gelling formulations were prepared. Altering the weight ratios of poloxamers affected the sol-to-gel transition temperature, mechanical and rheological properties and in vitro drug release. Desirable formulations gelled between 28 and 33°C providing sustained release of lidocaine over 48 h. CONCLUSION Thermosensitive gelling systems are promising for sustained drug release following patient administration and may be beneficial in addressing postoperative pain.

Inter-observer validation study of quantitative CT-osteodensitometry in total knee arthroplasty

IntroductionProximal tibial bone quality is an important factor in implant mechanical stability following total knee arthroplasty. Quantitative computed tomography (CT) osteodensitometry has been used to measure the change in bone density post-operatively. We sought to validate the inter-observer reliability of these measurements.MethodsSixteen CT scans were obtained on cadaveric lower limbs. Two observers independently obtained cortical and cancellous bone mineral density measurements from these scans. Eight CT scans were performed with prosthesis in situ and eight without. Results were compared using correlation coefficient and Bland–Altman analysis.ResultsThe study showed a high correlation coefficient of greater than 0.990 for all slices. The mean difference between observers falls within two standard deviations of the mean as seen on a Bland–Altman plot.InterpretationQuantitative CT-osteodensitometry is a reproducible method to measure bone mineral density changes about the knee and shows little variation between observers.