A numerical model to reproduce squeaking of ceramic-on-ceramic total hip arthroplasty. Influence of design and material.

BACKGROUND: Modern ceramic (CoC) bearings for hip arthroplasty (THA) have been used in younger patients who expect improved survivorship. However, audible squeaking produced by the implant is an annoying complication. Previous numerical simulations were not able to accurately reproduce in vitro and in vivo observations. Therefore, we developed a finite element model to: (1) reproduce in vitro squeaking and validate the model by comparing it with in vivo recordings, (2) determine why there are differences between in vivo and in vitro squeaking frequencies, (3) identify the stem's role in this squeaking, (4) predict which designs and materials are more likely to produce squeaking. HYPOTHESIS: A CoC THA numerical model can be developed that reproduces the squeaking frequencies observed in vivo. MATERIAL AND METHODS: Numerical methods (finite element analysis [ANSYS]) and experimental methods (using a non-lubricated simulated hip with a cementless 32mm CoC THA) were developed to reproduce squeaking. Numerical analysis was performed to identify the frequencies that cause vibrations perceived as an acoustic emission. The finite element analysis (FEA) model was enhanced by adjusting periprosthetic bone and soft tissue elements in order to reproduce the squeaking frequencies recorded in vivo. A numerical method (complex eigenvalue analysis) was used to find the acoustic frequencies of the squeaking noise. The frequencies obtained from the model and the hip simulator were compared to those recorded in vivo. RESULTS: The numerical results were validated by experiments with the laboratory hip simulator. The frequencies obtained (mean 2790Hz with FEA, 2755Hz with simulator, decreasing to 1759Hz when bone and soft tissue were included in the FEA) were consistent with those of squeaking hips recorded in vivo (1521Hz). The cup and ceramic insert were the source of the vibration, but had little influence on the diffusion of the noise required to make the squeaking audible to the human ear. The FEA showed that diffusion of squeaking was due to an unstable vibration of the stem during frictional contact. The FEA predicted a higher rate of squeaking (at a lower coefficient of friction) when TZMF™ alloy is used instead of Ti6Al4V and when an anatomic press-fit stem is used instead of straight self-locking designs. DISCUSSION: The current FEA model is reliable; it can be used to assess various stem designs and alloys to predict the different rates of squeaking that certain stems will likely produce. LEVEL OF EVIDENCE: Level IV in vitro study.

Scapulothoracic arthrodesis for patients who have fascioscapulohumeral muscular dystrophy.

In nine patients who had fascioscapulohumeral muscular dystrophy, thoracoscapular arthrodesis was done for the treatment of symptomatic winging of the scapula. The objective was to improve the use of the upper extremity in the performance of activities of daily living. Seven patients had a bilateral procedure. The active range of motion of the shoulder improved in all patients, with flexion increasing an average of 33 degrees and abduction, 25 degrees. Complications included pneumothorax, pleural effusion, atelectasis, fracture of the scapula, and pseudarthrosis. The length of follow-up averaged sixty-nine months, and the results did not deteriorate with time.

Morphology and functional anatomy of the first dorsal interosseous muscle of the hand.

The aim of this study, based upon anatomical and electrophysiological evaluation, was to identify the relationship between the morphology and physiology of the first dorsal interosseous muscle of the hand, which should be distinguished from the other dorsal interosseous muscles of the hand. Its morphology, distal attachments and physiology have been subject of numerous studies yielding conflicting results. The study reported herein was made on the basis of anatomical and electrophysiological investigation. Most of the dissections (20/34) were made on fresh specimens. Results of this study confirm the existence of the deep and superficial heads of the 1st dorsal interosseous muscle. The muscle is attached distally to the palmar plate of the metacarpophalangeal joint, the lateral tubercle of the base of the proximal phalanx of the index and the interosseous hood. Conversely, the muscle did not show any attachment to the oblique radial wing of the extensor apparatus in our dissections. The deep head of the muscle causes mainly flexion pinch between thumb and index superficial head abduction. Within the complex physiology of the various types of apposition of thumb and index, the dorsal interosseous muscle acts as a stabilizer. The results of electrophysiological study confirmed most of the interpretations deduced from morphological investigation.