Abstract

With its high wear and corrosion resistance, CoCrMo alloy has been widely used for metal-on-metal total hip replacements (THRs). However, the use of the metal-on-metal implants has dropped substantially as a result of several alerts issued by the Medicines and Healthcare products Regulatory Agency (MHRA) due to concern on metal ion release [1]. However, some of the first generation of metal-on-metal THRs have lasted for more than 20 years [2]. It is far from clear why some MoM joints have survived, while other failed. It is known that dynamic changes occur at the metal surface during articulation. For example, a nanocrystalline layer has been reported on the topmost surface of both in vivo and in vitro CoCrMo THRs [3, 4] but it is not known whether this layer is beneficial or detrimental.

The current work focuses on the sub-surface damage evolution of explanted MoM hips, which is compared to in vitro tested CoCrMo hip prostheses. Site-specific TEM cross-section of both in vivo and in vitro CoCrMo samples were prepared by focused ion beam (FIB) in situ lift-out method (Quanta 200 3D with Omniprobe, FEI, the Netherlands). TEM of the FIB specimens was performed on various microscopes. Routine bright field imaging was performed on a Tecnai 20 (FEI, the Netherland) operating at 200 kV, while high resolution transmission electron microscopy (HRTEM) of the nanocrystalline layer and other surface species was undertaken on a Jeol 2010F (Jeol, Japan) operating at 200 kV.

A nanocrystalline layer (which was not present on the starting surfaces) was observed on both explanted in vivo and in vitro tested materials. For the explanted joints, the nanocrystalline layer was thin (a few 100 nm) and the extent did not appear to correlate with the local wear rate. For in vitro samples, the nanocrystalline layer was thicker (up to micron). HRTEM from this layer are shown in Fig. 1 and Fig. 2. The nanocrystallite size was ∼5 nm and appeared to be a mixture of face centred cubic and hexagonal close packed phases. The formation of the nanocrystalline layer and its correlation with wear behaviour are discussed.

This is a case series report on the outcomes of patients that have received ORIF of their calcaneal fractures at Craigavon Hospital, Northern Ireland, for the first 2 years since it opened. It is a one surgeon series.

INTRODUCTION: Structural changes to the intervertebral disc (IVD) in the form of anular lesions are a feature of IVD degeneration. Degeneration has been related to changes in the mechanical function of the IVD. This study determined the mechanical effect of individual concentric tears, radial tears and rim lesions of the anulus in an in vitro experiment.

METHODS: The lumbar spines from five sheep were taken post mortem and divided into three motion segments. The disc body units were tested on a robotic testing facility, using position control, in flexion/extension, lateral bending and axial rotation. Concentric tears, radial tears and rim lesions were experimentally introduced and the motions repeated after the introduction of each lesion. The mechanical response after the lesion creation was compared to the undamaged response to assess the mechanical effect of each lesion.

RESULTS: It was found that an anterior rim lesion reduced the peak moment resisted by the disc in extension, lateral bending and axial rotation. Concentric tears and radial tears did not affect the peak moment resisted, however, radial tears reduced the hysteresis of response in flexion/extension and lateral bending. The neutral zone was not affected by the presence of IVD lesions.

DISCUSSION: These results show that rim lesions reduce the disc’s ability to resist motion. Radial tears change the hysteresis of response indicating an altered stress distribution in the disc. These changes may lead to overloading of the spinal ligaments, muscles and zygapophysial joints, possibly damaging these structures. This suggests a mechanism for a cycle of degeneration that is instigated by small changes in the mechanical integrity of the IVD.

Introduction: the neutral zone is defined as a region of no or little resistance to motion in the middle of an intervertebral joint’s range of movement. Previous studies have used quasistatic loading regimes that do not model physiological activity¹. The aim of the present study was to assess experimentally the existence of the neutral zone of intervertebral joints during spinal motion in flexion/extension, lateral bending and axial rotation during physiological movements simulated using a robotic testing facility. Sheep intervertebral joints were used as they have been shown to exhibit similar mechanical behaviour to human joints².

Methods: five spines from mature sheep were used. Three specimens were tested from each spine to simulate human l1/2, l3/4 and l4/5 intervertebral joints. The robotic facility enabled the testing regime to be defined for each individual joint based on its geometry. The joints were tested by cycling through the full range of physiological movement in flexion/extension, lateral bending and axial rotation.

Results: a neutral zone was found to exist during dynamic movements only in flexion/extension. The results were equivocal for lateral bending and suggested that a neutral zone does not exist in axial rotation. The zygapophysial joints were shown to be significant in determining the mechanics of the intervertebral joints as their removal increased the neutral zone in all cases. A criterion for defining the size of the neutral zone was proposed.

Conclusions: a neutral zone exists in flexion/extension during dynamic movements of intervertebral joints. This has important implications for the muscular control of the spine consisting of several intrinsically lax joints stacked on one another.