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BACKGROUND: To assess whether lumbar pedicle screw placement with a minimally invasive surgical (MIS) vs. open technique imparts different biomechanical parameters and thus may affect failure rates. METHODS: Human cadaveric disarticulated lumbar vertebrae 1–5 were stabilised in cement. Pedicle screws were inserted either via the ‘MIS’ or ‘open’ technique, based on previously described anatomical landmarks. Each vertebra had one ‘MIS’ and one ‘open’ technique screw. Specimens were tested with an Instron mechanical testing machine, positioned to allow for testing of direct coaxial force. Load was applied until failure occurred, and load-displacement curves generated for each screw. RESULTS: Average failure load was found to be 685±399 N for MIS, versus 661±323 N for open technique (P=0.75). The average ultimate failure load was 748±421 N for MIS, versus 772±326 N for open (P=0.74). Average displacement until failure was 0.95±0.49 mm for MIS as compared to 0.95±0.62 mm for open (P=0.996). Axial stiffness was 936±217 N/mm for MIS and 1,016±263 N/mm for open (P=0.19). Average work required to result in failure was 0.84±1.09 J for MIS and 0.82±1.05 J for open (P=0.94). CONCLUSIONS: There was no significant difference in the biomechanical properties of the MIS as compared with open lumbar pedicle screws, when tested until failure under direct coaxial force. The clinical implication may be that there is no significant advantage in the biomechanical properties of MIS versus open lumbar pedicle screw insertion techniques.

As the use of glenoid suture anchors in arthroscopic and open reconstruction, for instability after Bankart lesions of the shoulder, increases, an emerging problem has been the incidence of glenoid rim fractures through suture drill holes. Very little is known regarding the effect of the Hill–Sachs lesion on the glenoid's susceptibility to fracture and how drill hole location can further affect this. This study used finite element modeling techniques to investigate the risk of fracture of the glenoid rim in relation to variable sized Hill–Sachs defects impacting on the anterior glenoid edge with suture anchor holes placed in varying positions. The distribution of Von Mises (VM) stresses and the factor of safety (FOS) for each of the configurations were calculated. The greatest peak in VM stresses was generated when the glenoid was loaded with a small Hill–Sachs lesion. The VM stresses were lessened and the FOS increased (reducing likelihood of failure) with increasing size of the Hill–Sachs lesion. Placement of the suture drill holes at 2 mm from the glenoid rim showed the highest risk of failure; and when combined with a medium sized Hill–Sachs lesion, which matched the central line of the drill holes, a potentially clinically significant configuration was presented. The results of this study are useful in assisting the surgeon in understanding the interaction between the Hill–Sachs lesion size and the placement of suture anchors with the purpose of minimizing the risk of subsequent rim fracture with new injury.

The occurrence of audible squeaking in some patients with ceramic-on-ceramic (CoC) hip prostheses is a cause for concern. Considering multifactor contributing to this phenomenon, many studies have been conducted over the last decade. Great efforts have been put on understanding the mechanics of the hip squeaking to gain a deep insight into factors resulting in sound emission from hip articulation. Disruption of fluid-film lubrication and friction were reported as main potential causes of hip squeaking, while patient and surgical factors as well as design and material of hip implants were identified as affecting factors. This review article therefore summarised the recent available literature on this subject to provide a platform for future developments. Moreover, high wear rates and ceramic liner fracture as viable consequences of hip squeaking were discussed., The first author gratefully acknowledges Macquarie University for International Macquarie University Research Excellence Scholarship (iMQRES)-No. 2010017. The second author would like to thank to the Portuguese Foundation for Science and Technology through the project UID/EEA/04436/2013.

The upper limb undergoes a large range-of-motion (ROM); and therefore, the mechanical testing techniques for the corresponding joint implants need to reflect these movements. The literature shows that aseptic loosening and implant instability are the main shoulder implant related reasons for revision surgery. Aseptic loosening occurs when excessively high stresses lead to degradation of the articular surface of the implant component, resulting in generation of wear debris and implant instability that is mostly caused by eccentric loading. Therefore shoulder implant testing generally falls under two different testing modes: wear testing and stability testing. Both anatomic and reverse type shoulder implants have been tested using a variety of different testing regimes including static and dynamic as well as constant loading, variable loading, and physiological loading. There are currently no standard mechanical testing protocols available for Elbow implants, with only a few reports of varus-valgus laxity test methods found in the literature.

It is still unclear how a vertebral fracture should be stabilised and strengthened without endangering the remaining intact bone of the augmented vertebra or the adjacent vertebrae. Numerical modelling may provide insight. To date, however, few finite element (FE) spine models have been developed which are both multi-segmental and capture a more complete anatomy of the vertebrae. A 3-D, two-functional unit, CT-based, lumbar spine, FE model was developed and used to predict load transfer and likelihood of fracture following balloon kyphoplasty. The fractured anterior wall and injected cement were modelled in a two-functional spinal unit model with osteoporotic bone properties. Parameters investigated included: cement stiffness, cement volume and height restoration. Models were assessed based on stresses and a user-defined fracture-predicting field. Augmentation altered the stress distribution; shielding was dependent on positioning of the cement; and fracture algorithm found incomplete height restoration to increase the likelihood of fracture, particularly in adjacent vertebrae.

A ceramic-on-ceramic (CoC) hip prosthesis with clearance is modeled as a multibody dynamics system for the purpose of studying hip squeaking. A continuous contact force model provides the intrajoint forces developed at the hip joint. Friction effects due to the relative motion are also considered. A FFT analysis of the audible sounds from CoC hip acceleration is carried out to analyze hip squeaking. The effects of friction, hip implant size, and the head initial position on hip squeaking and the trajectory of femoral head are analyzed and discussed. It was shown that the causes of hip squeaking are stick/slip, friction-induced vibration, and the femoral head angular speed and force changes. [DOI: 10.1115/1.4028109]

Wear plays a key role in primary failure of artificial hip articulations. Thus, the main goal of this work is to investigate the influence of friction-induced vibration on the predicted wear of hard hip arthroplasties. This desideratum is reached by developing a three-dimensional multibody dynamic model for a hip prosthesis taking the spatial nature of the physiological loading and motion of the human body into account. The calculation of the intra-joint contact forces developed is based on a continuous contact force approach that accounts for the geometrical and materials properties of the contacting surfaces. In addition, the friction effects due to the contact between hip components are also taken into account. The vibration of the femoral head inside the cup associated with stick-slip friction, negative-sloping friction and dynamic variation in intra-joint contact force has been also incorporated in the present hip articulation model. The friction-induced vibration increases the sliding distance of the contact point between the head and cup surfaces by altering its micro and macro trajectories, and consequently affects the wear. In the present work, the Archard’s wear law is considered and embedded in the dynamic hip multibody model, which allows for the prediction of the wear developed in the hip joint. With the purpose of having more realistic wear simulation conditions, the geometries of the acetabular cup and femoral head are updated throughout the dynamic analysis. The main results obtained from computational simulations for ceramic-on-ceramic and metal-on-metal hip prostheses are compared and validated with those available in the best-published literature. Finally, from the study performed in the present work, it can be concluded that that an important source of the high wear rates observed clinically may be due to friction-induced vibration., The first author gratefully acknowledges Macquarie University for his International Macquarie University Research Excellence Scholarship (iMQRES)-No. 2010017. The second author would like to thank the Portuguese Foundation for Science and Technology (FCT) through the project UID/EEA/04436/2013.

Wear can influence the lifetime and performance of implants and has been found to be a key factor in primary failure of artificial hip joints. The present study aims to present a spatial multibody dynamic model to predict wear in ceramic- on-ceramic hip implants. The problem was formulated by developing a spatial multibody dynamic model of a hip prosthesis taking three-dimensional physiological loading and motion of the human body into account. Then, the Archard wear model was integrated into the dynamic calculation of the hip implant to predict wear. Additionally, geometries of the cup and head were updated throughout the simulation to generate a more realistic wear simulation. The results were validated against current literature. Finally it was illustrated that friction-induced vibration caused excessive wear of hip implant components.

The main objective of this work is to study the effect of friction-induced vibration and contact mechanics on the maximum contact pressure and moment of artificial hip implants. For this purpose, a quasi-static analysis and a multibody dynamic approach are considered. It is shown that the multibody dynamic model is effective at predicting contact pressure distribution and moment of hip implants from both accuracy and time-consuming points of view. Finally, from the computational simulations performed, it can be observed that the friction-induced vibration influences the contact pressure and the moment in hip implants by introducing an oscillating behaviour in the system dynamics., Fundação para a Ciência e a Tecnologia (FCT)

In vitro mechanical testing of orthopedic implants provides information regarding their mechanical performance under simulated biomechanical conditions. Current in vitro component stability testing methods for reverse shoulder implants are based on anatomical shoulder designs, which do not capture the dynamic nature of these loads. With glenoid component loosening as one of the most prevalent modes of failure in reverse shoulder replacements, it is important to establish a testing protocol with a more realistic loading regime. This paper introduces a novel method of mechanically testing reverse shoulder implants, using more realistic load magnitudes and vectors, than is currently practiced. Using a custom made jig setup within an Instron mechanical testing system, it is possible to simulate the change in magnitude and direction of the joint load during arm abduction. This method is a step towards a more realistic testing protocol for measuring reverse shoulder implant stability.