In vivo glenohumeral contact forces—Measurements in the first patient 7 months postoperatively

[Partial weight-bearing in rehabilitation. Strategies for instruction and limitations]

Following trauma or surgery on the musculoskeletal system the primary aim is always as complete a restitution of mobility as possible. By mobilization with partial weight-bearing this is possible. The preferred way of teaching partial weight-bearing is the use of conventional bathroom scales. This method proves to be simple as well as time and cost-saving, but the transferability to the patient's daily life is questionable. Training and control of partial weight-bearing under dynamic conditions, such as normal walking, and walking up and down stairs seem to be very important. Different investigations have shown that the minority of subjects recruited could manage to maintain the given load of partial weight-bearing. Furthermore, the actual resulting moments within the joints, caused by muscles, fascia and tendons, are not considered in presets of partial weight-bearing, as only external forces (ground reaction forces) are measured. However, the problems in teaching partial weight-bearing have to be contrasted with the as yet unexplained issue of postoperative partial versus full weight-bearing.

Glenohumeral contact kinematics in patients after total shoulder arthroplasty

BACKGROUND

Knowledge of in vivo glenohumeral joint contact mechanics after total shoulder arthroplasty may provide insight for the improvement of patient function, implant longevity, and surgical technique. The objective of this study was to determine the in vivo glenohumeral joint contact locations in patients after total shoulder arthroplasty. We hypothesized that the glenohumeral joint articular contact would be centered on the glenoid surface because of the ball-in-socket geometric features of the implants.

METHODS

Dual-plane fluoroscopic images and computer-aided design models were used to quantify patient-specific glenohumeral articular contact in thirteen shoulders following total shoulder arthroplasty. The reconstructed shoulder was imaged at arm positions of 0 degrees, 45 degrees, and 90 degrees of abduction (in the coronal plane) and neutral rotation and at 90 degrees of abduction with maximum internal and external rotation. The patients were individually investigated, and their glenohumeral joint contact centroids were reported with use of contact frequency.

RESULTS

In all positions, the glenohumeral joint contact centroids were not found at the center of the glenoid surface but at an average distance (and standard deviation) of 11.0 +/- 4.3 mm from the glenoid center. Forty (62%) of the sixty-five total contact occurrences were found on the superior-posterior quadrant of the glenoid surface. The position of 0 degrees of abduction in neutral rotation exhibited the greatest variation of quadrant contact location; however, no contact was found on the superior-anterior quadrant of the glenoid surface in this position.

CONCLUSIONS

In vivo, glenohumeral joint contact after total shoulder arthroplasty is not centered on the glenoid surface, suggesting that kinematics after shoulder arthroplasty may not be governed by ball-in-socket mechanics as traditionally thought. Although contact locations as a function of arm position vary among patients, the superior-posterior quadrant seems to experience the most articular contact in the shoulder positions tested.

Failure mechanism of the all-polyethylene glenoid implant

Fixation failure of glenoid components is the main cause of unsuccessful total shoulder arthroplasties. The characteristics of these failures are still not well understood, hence, attempts at improving the implant fixation are somewhat blind and the failure rate remains high. This lack of understanding is largely due to the fundamental problem that direct observations of failure are impossible as the fixation is inherently embedded within the bone. Twenty custom made implants, reflecting various common fixation designs, and a specimen set-up was prepared to enable direct observation of failure when the specimens were exposed to cyclic superior loads during laboratory experiments. Finite element analyses of the laboratory tests were also carried out to explain the observed failure scenarios. All implants, irrespective of the particular fixation design, failed at the implant-cement interface and failure initiated at the inferior part of the component fixation. Finite element analyses indicated that this failure scenario was caused by a weak and brittle implant-cement interface and tensile stresses in the inferior region possibly worsened by a stress raiser effect at the inferior rim. The results of this study indicate that glenoid failure can be delayed or prevented by improving the implant/cement interface strength. Also any design features that reduce the geometrical stress raiser and the inferior tensile stresses in general should delay implant loosening.

Is there an association between a low acromion index and osteoarthritis of the shoulder?

The aim of this study was to evaluate the relationship between a low acromion index and osteoarthritis of the shoulder. Three patient groups were used: (I) instability, n = 53; (II) calcifying tendonitis, n = 109; and (III) osteoarthritis, n = 120. Standardised digital X-rays were evaluated from the true anteroposterior and axillary views. Joint space width at three levels in each plane and the size of humeral osteophytes were measured and osteoarthritis was graded according to Samilson. The acromion index was calculated according to Nyffeler et al. (J Bone Joint Surg Am 88:800-805, 2006) in the true anteroposterior view. There were two independent investigators. Interobserver reliability was excellent for all measurements in the anteroposterior (AP) projection but inferior in the axillary projections, especially in group III. The mean acromion index was 0.64 ± 0.07 in group I, 0.64 ± 0.08 in group II and 0.73 ± 0.12 in group III. The acromion index was not correlated with the joint space width nor with the size of the osteophytes or the Samilson grading in group III. The data of the study did not show a significant association between a low acromion index and typical signs of osteoarthritis at the shoulder. The theoretical concept of a small acromion index associated with the development of osteoarthritis of the shoulder is not supported.

Análise por dinâmica inversa, um complemento da avaliação fisioterapêutica do ombro

O objetivo do estudo é caracterizar as forças em atuação na articulação do ombro durante o movimento de elevação do membro superior no plano escapular por meio do método de dinâmica inversa, aqui sugerido como um meio complementar da avaliação fisioterapêutica. Esse método permite determinar os picos de momento proximal resultante (MPR) e da força proximal resultante (FPR) do ombro durante o movimento, possibilitando assim a avaliação objetiva das cargas impostas à articulação. Participaram do estudo 21 indivíduos do sexo masculino, cuja movimentação do ombro foi analisada por videogrametria em três diferentes situações de carga: sem carga, com peso livre e com resistência elástica. Um modelo matemático tridimensional foi utilizado para o cálculo do MPR e da FPR, permitindo caracterizar a evolução dessas variáveis ao longo da elevação do membro superior nas três situações de carga nos eixos póstero-anterior, caudal-cranial e médio-lateral, determinando seus respectivos picos. O método da dinâmica inversa revelou-se capaz de fornecer informações objetivas sobre as cargas impostas à articulação do ombro nas diversas amplitudes e situações de carga do movimento estudado, podendo tais informações servir como uma base concreta no planejamento de um programa de reabilitação do ombro.

Numerical modelling of the shoulder for clinical applications

Research activity involving numerical models of the shoulder is dramatically increasing, driven by growing rates of injury and the need to better understand shoulder joint pathologies to develop therapeutic strategies. Based on the type of clinical question they can address, existing models can be broadly categorized into three groups: (i) rigid body models that can simulate kinematics, collisions between entities or wrapping of the muscles over the bones, and which have been used to investigate joint kinematics and ergonomics, and are often coupled with (ii) muscle force estimation techniques, consisting mainly of optimization methods and electromyography-driven models, to simulate muscular action and joint reaction forces to address issues in joint stability, muscular rehabilitation or muscle transfer, and (iii) deformable models that account for stress-strain distributions in the component structures to study articular degeneration, implant failure or muscle/tendon/bone integrity. The state of the art in numerical modelling of the shoulder is reviewed, and the advantages, limitations and potential clinical applications of these modelling approaches are critically discussed. This review concentrates primarily on muscle force estimation modelling, with emphasis on a novel muscle recruitment paradigm, compared with traditionally applied optimization methods. Finally, the necessary benchmarks for validating shoulder models, the emerging technologies that will enable further advances and the future challenges in the field are described.

Development of a 4-axis load cell used for lumbar interbody load measurements

Numerous studies have assessed lumbar interbody fusion, but little data from direct interbody load measurements exists. This manuscript describes an interbody fusion cage with integrated 4-axis load cell that can simulate implant heights of 13, 15, 17, 19 and 21 mm. The calibrated load cell was accurate to within 7.9% for point compressive loads over the central 8 mm x 8 mm region, but up to 26.8% for eccentric loads on the outer 16 mm x 16 mm rim of the device (although typically errors were less than half). Anterior-posterior shear and lateral shear loads did not affect compressive load measurement (<1.0% and <3.5%, respectively). Moments calculated from 4 load sensing corner pillars demonstrated errors below 2.3% in lateral bending and 2.1% in flexion-extension. Although this device does not have the accuracy of other much larger corpectomy implants, it incorporates four channels of load and simulates multiple implant heights, making for a favorable comparison in this restricted space. This device has immediate use in cadaveric testing, providing data previously not attainable, and serves as a novel technological step towards an implantable interbody device with multi-axis load sensing capability. As per the authors' knowledge, no such device has previously been described.

Effect of rotator cuff dysfunction on the initial mechanical stability of cementless glenoid components

The functional outcome of shoulder replacement is related to the condition of the rotator cuff. Rotator cuff disease is a common problem in candidates for total shoulder arthroplasty; this study relates the functional status of the rotator cuff to the initial stability of a cementless glenoid implant. A 3D finite element model of a complete scapula was used to quantify the effect of a dysfunctional rotator cuff in terms of bone-implant interface micromotions when the implant is physiologically loaded shortly after surgery. Four rotator cuff conditions (from fully intact to progressively ruptured rotator cuff tendons) as well as two bone qualities were simulated in a model. Micromotions were significantly larger in the worst modeled cuff dysfunction (i.e. the supraspinatus and infraspinatus tendons were fully dysfunctional). Micromotions were also significantly different between conditions with healthy and poor bone quality. The implant's initial stability was hardly influenced by a dysfunctional supraspinatus alone. However, when the infraspinatus was also affected, the glenohumeral joint force was displaced to the component's rim resulting in larger micromotions and instability of the implant.

Validation of a musculoskeletal model of wheelchair propulsion and its application to minimizing shoulder joint forces

The majority of manual wheelchair users (MWUs) will inevitably develop some degree of shoulder pain over time. Previous research has suggested a link between the shoulder joint forces associated with the repetition of wheelchair (WC) propulsion and pain. The objective of this work is to present and validate a rigid-body musculoskeletal model of the upper limb for calculation of shoulder joint forces throughout WC propulsion. It is anticipated that when prescribing a WC, the use of a patient-specific computational model will aide in determining an axle placement in which shoulder joint forces are at a minimum, thus potentially delaying or reducing the shoulder pain that so many MWUs experience. During the validation experiment, 3 subjects (2 individuals with paraplegia and one able-bodied individual) propelled a WC at a self-selected speed, during which, kinematics, kinetics, and electromyography (EMG) activity were measured for the contact phase of 10 consecutive push strokes. The measured forces at the push rim and the 3-D propulsion kinematics drove the model, and the computationally calculated muscle activities were compared with the experimental muscle activities, resulting in an average mean absolute error (MAE) of 0.165. Further investigation of the shoulder joint forces throughout propulsion demonstrate the effect of axle placement on the magnitude of these forces. The present work serves to validate the patient-specific upper limb model for use as a prescriptive tool for fitting a subject to their WC. Minimizing joint forces from injury onset may prolong a MWU's pain-free way of life.

Simulated joint and muscle forces in reversed and anatomic shoulder prostheses

Reversed shoulder prostheses are increasingly being used for the treatment of glenohumeral arthropathy associated with a deficient rotator cuff. These non-anatomical implants attempt to balance the joint forces by means of a semi-constrained articular surface and a medialised centre of rotation. A finite element model was used to compare a reversed prosthesis with an anatomical implant. Active abduction was simulated from 0 degrees to 150 degrees of elevation. With the anatomical prosthesis, the joint force almost reached the equivalence of body weight. The joint force was half this for the reversed prosthesis. The direction of force was much more vertically aligned for the reverse prosthesis, in the first 90 degrees of abduction. With the reversed prosthesis, abduction was possible without rotator cuff muscles and required 20% less deltoid force to achieve it. This force analysis confirms the potential mechanical advantage of reversed prostheses when rotator cuff muscles are deficient.