The design of the femoral prosthesis in cementless total hip arthroplasty is known to affect the initial strains in the cortex during implantation and in the early postoperative time period. High strains have a direct influence on periprosthetic fracture. This study compares the existing ABGII stem, which is proximally coated with a grit blasted titanium surface with hydroxyapatite coating with a prototype that has a rougher titanium plasma spray proximal coating. The Australian National Joint registry results 2011 reported the ABG2 femoral component cumulative percent revision (CPR) of 6.5 (93.5% survival), which compares favourably with equivalent stems with 10 year CPR data such as the Taperloc 6.6 and Corail 7.3.

Six pairs of fresh-frozen cadaveric femurs were mounted in blocks according to ISO guidelines in single leg stance setup. Five strain gauges were attached around the neck of the femur and then prepared according to routine operative techniques to accept the femoral prosthesis. Cortical strains were measured during insertion of the prosthesis with an instrumented mallet attached to an accelerometer. Subsequently, force-displacement readings were taken during cyclical loading on a servo-hydraulic machine and finally the stems were tested to failure.

Our results showed significantly less strain during cyclical loading of the stem with increased surface roughness (p < 0.05). They also showed no significant differences loads/strains during impaction (p = 0.159), no significant difference in micromotion (p = 0.148) and no significant difference in load-to-failure (p = 0.37).

Introduction

Metal on metal (MoM) bearings have been dealt a severe blow in the past few years. The release of metal ions may have arisen from corrosion, wear, or a combination of the two. Edge loading due to implant malposition is thought to cause a failure of lubrication and to contribute to excessive wear and increased metal ion release [1]. Literature reports aseptic lymphocytic vasculitis-associated lesions (ALVAL) are associated with a variety of failures which occur to some degree in all implanted metal femoral components [2, 3]. Moreover, Willert et al [4] has described ALVAL in non-MoM bearing designs too. This paper has investigated the metal ion release due to total hip replacement (THR), Hip Resurfacing (HR) and total knee replacement (TKR).

Purpose

We present results of high intensity focussed ultrasound (HIFU) therapy on 2 patients with recurrent sacrococcygeal chordoma with the aim to promote recruitment of patients into a larger clinical trial

A finite element study was carried out to compare the performance of a three-hole locking plate with angled screws to the ‘gold-standard’ four-hole hip plate. Two cases of the three-hole hip plate were examined; (a) three screws and (b) two screws (most proximal and most distal).

A 3D model of the proximal femur was constructed from CT scans. A 3D CAD model of the four-hole hip plate was also created. The three-hole hip plate was then created from the four-hole implant in a way that it was possible to switch between all three models by activating/deactivating sections and/or switching material properties. A single common finite element model was generated, and a static analysis of each model variation was then performed in two steps using ABAQUS/standard. In the first, screws were pre-tensioned up to 150N. In the second, loads corresponding to stair climbing were applied.

Forces in the screws, permitted to change in the second step, were examined and compared. Maximum principal stresses in the bone were also examined, with a focus on the stresses in the bone at the end of the plate in each model. The highest tensile force was in the proximal screw of the three-hole plate with three screws, followed by the most distal screw in the standard four-hole plate. This suggests that the risk of screw pull-out is highest at the proximal screw of the three-hole hip plate with three screws.

A comparison of the forces in the distal screws for all cases shows that the highest tensile force was in the four-hole plate, followed by the three-hole plate with two screws. The lowest was the three-hole plate with three screws, which was in compression at full load. The maximum tensile stresses in the bone at the end of the plate were greatest for the standard four-hole hip plate, followed by the three-hole plate with two screws and then the three-hole plate with three screws. This indicates that the risk of bone fracture at the end of the plate is lowest for the three-hole hip plate with three screws.

The risk of bone fracture is significantly lower for the three-hole hip plate, with either two or three screws, compared to the ‘gold-standard’ four-hole hip plate. This is partially offset by a small increase in the risk of screw pull out (in the proximal rather than the distal screw).

Squeaking ceramics bearing surfaces have been recently recognised as a problem in total hip arthroplasty. The position of the acetabular cup has been alluded to as a potential cause of the squeaking, along with particular combinations of primary stems and acetabular cups. This study has used the finite element method to investigate the propensity of a new large diameter preassembled ceramic acetabular cup to squeaking due to malpositioning.

A verified three-dimensional FE model of a cadaveric human pelvis was developed which had been CT scanned, and the geometry reconstructed; this was to be used to determine the behaviour of large diameter acetabular cup system with a thin delta ceramic liner in the acetabulum. The model was generated using ABAQUS CAE pre-processing software. The bone model incorporated both the geometry and the materials properties of the bone throughout based on the CT scan. Finite element analysis and bone material assignment was performed using ABAQUS software and a FORTRAN user subroutine. The loading applied simulated edge loading for rising from a chair, heel-strike, toe off and stumbling.

All results of the analysis were used to determine if the liner separated from the shell and if the liner was toggling out of the shell. The results were also examined to see if there was a propensity for the liner to demobilise and vibrate causing a squeaking sound under the prescribed loading regime.

This study indicates that there is a reduction in contact area between the ceramic liner and titanium shell if a patient happens to trip or stumble. However, since the contact between the liner and the shell is not completely lost the propensity for it to squeak is highly unlikely.

Introduction: Periprosthetic bone resorption following total knee arthroplasty (TKA) is becoming a clinical concern. Decrease in bone quality jeapordises implant fixation, consequently leading to revision surgery. It has been suggested that a reduction in the local stress distribution may cause a decrease in bone mineral density (BMD). Computational bone remodelling has been used previously to predict bone adaptation in total hips. However, little has been reported on its use in TKA remodelling simulations. The aim of this study was to simulate the bone remodelling response of the femur and tibia following TKA, using an adaptive bone remodelling algorithm combined with the finite element (FE) method.

Methods: 3D femur and tibia models were constructed from human cadaveric computed tomography images. Total knee implant geometries were used to reconstruct the knee joint.(RBK, Global Orthopaedic Technology, Australia). Both the femur and the tibia models were loaded at 45% gait cycle for normal walking gait using loads based on Taylor et al. A strain-adaptive remodelling algorithm was used to predict the remodelling behaviour of the femur and tibia following TKA. Analysis was performed using ABAQUS. Virtual DEXA images were generated from the FE models at predetermined time-points, BMD gain and loss were also assessed both quantitatively and qualitatively.

Results: There was an increase and decrease in BMD for the femur and tibia models. BMD loss in the femur was predominantly experienced around the pegs and the distal femoral regions. Femoral BMD gain was displayed around the edges of the bone-implant interface, with higher activity at the anterior-medial and posterior-lateral aspects. BMD gain in the tibia was predominantly at the inferior end of the tibial tray’s keel, with the bone mass tending towards the medial aspect. Some bone gain was displayed on the medial side, surrounding the pegs and at the cortex. There was BMD loss on the lateral aspect of the tibia.

Discussion: The adaptive bone remodelling algorithm has shown a good correlation with clinical findings. Reports of clinical and FE studies have shown that for cemented knees, most bone loss occurs at the distal femoral region, especially at the anterior aspect. It has been reported that in the tibia there is generally an over-all decrease in BMD in the proximal tibia and increase below the keel. This is in accordance with our predictions. BMD gain was found to be more predominant on the medial aspect. This may be due to the more medially inclined loading ratio, which affects the stress distribution within the bone. BMD gain in the tibia is shown to follow a path, which starts at the bottom of the keel and tends medially towards the tibial cortex. This illustrates the inherent tendency of load transfer to follow along the stiffest structural path.

Introduction: High ion release along with bone resorption at the bone/implant interface is still a problem, leading to pain, poor function and the possibility of bone fracture. Treatment of a loose implant is not easy and can lead to less than satisfactory revision surgery. The reason for ion release, loosening or periprosthetic fracture of an implant is multifactorial. One factor for ion release that has been reported is inclination angle. Another can be the version angle of the implant and subjecting it to an abnormal loading environment. Few studies have been reported in the literature on hip resurfacing performance based on implant orientation. More studies are required into investigating the use of this predictive technique in orthopaedics to investigate the bearing behaviour and potential ion release due to implant surgical positioning. In this study we modeled a number of different version angles and investigated the contact area, stress and wear characteristics using the finite element method.

Methods: CT scans were used to reconstruct the part of the femur and pelvic geometry. A 3D finite element mesh was created using PATRAN (MSC Software, Santa Ana, CA). The femur loading was taken at peak load position of the gait cycle. The loading was applied to the femur and pelvis was fixed. Material properties were applied using the Hounsfield units from the CT file. Two models were generated, a preoperative and a postoperative state model. The post operative model was reconstructed using the Birmingham Hip Replacement (BHR) system (Smith & Nephew Inc, Memphis, TN). The BHR acetabular cup was oriented at different anteversion angles (5°, 30° & 45° to the saggital plane) to investigate the contact mechanics between the head and cup. Serum ion levels were taken from 12 patients and the change in ion levels over the first 12 month period were analysed statistical to investigate the correlation with anteversion angle. Radiographs from the same patients were analysed to determine the cup anteversion angle using image analysis and edge matching techniques.

Results: The contact areas increased with increasing anteversion angle, 137.3, 165.3 and 169.9mm2 respectively. As a consequence, the contact pressure decreased. The change in ion levels for the patients over the first 12 month period correlated significantly (p< .05) with the anteversion angle using Pearson’s r test.

Discussion: Statistical analysis showed a good Pearson’s correlation of anteversion angle to a change in serum ion levels, 0.867 and 0.734 with p values of 0.001and 0.012 respectively. Acetabular version angle appears to be, at the least, important in determining serum metal ion levels and in evaluating causes of metallosis, the influence of anteversion angle needs to be considered when using metal on metal bearing technology when placing the cup in the acetabulum.

Geometric and material changes in the femoral neck following hip resurfacing have been linked to femoral neck fractures.

This study developed a unique method to determine the level of influence of the implant stem on the structural changes in the femoral neck following surgery.

A 3D femur model was generated using CT-images. The finite-element model was meshed using 10-noded tetrahedral elements. An ASR hip-resurfacing component (Depuy International, Leeds) was implanted into the femur in load sensitive position. A strain-adaptive bone-remodelling algorithm was used to determine the bone-remodelling behaviour of the femur over a minimum of 2-year period.

Following the analysis, the material properties and stresses in the neck region were mapped onto a cubic mesh, which simulated a CT stack. Moments of inertia, bending moments and shear was calculated for each slice along the neck of femur. These were compared to the pre-operative model.

Bone mineral density changes in the neck region were observed following implantation due to the changes in moments of inertia, bending moments and shear loading.

A method to determine the effect of implantation on the geometric and densitychanges in the femoral neck following resurfacing was developed. This methodology has shown that implant stem geometry affects the load transfer to the femur and the adaptive behaviour of the femoral neck. This will influence the structural integrity of the femoral neck and the long-term clinical outcome of the hip resurfacing component.

A recurrent fracture rate after vertebroplasty and balloon kyphoplasty is as high as 20%. Biomechanically, it has not been proven that refracture rate is due to the cement stiffness alone. This finite-element study investigated effects of cement-stiffness, bone-quality, cement-volume and height-restoration in treatment of vertebral compression fractures using balloon kyphoplasty.

A finite-element model of the lumbar spine was generated from CT-scans. The model comprised of two functional spinal-units, consisting of L2-L4 vertebral bodies, intervertebral-discs, and spinal ligaments. Cement volumes modelled were in the order of 15% and 30% of total vertebral body (VB) volume. Spinal fracture was modelled as being reduced and height of VB was restored. Kyphoplasty was performed. Three different bone qualities were modelled: healthy, osteopenic, osteoporotic. A compressive load was applied to the proximal endplate of L2. An anterior shift of the centre-of-gravity of upper body was simulated by increasing the moment arm of the applied load.

All results of the analysis were compared back to an intact spinal model of the same region under the same loading regime. All parameters affected the mechanical behaviour of the spine model, although changing the bone quality from normal to osteoporotic resulted in the least change. The cement stiffness was initially modelled with an elastic modulus between 0.5GPa and 2GPa. The results showed small differences relative to intact case in the lower modulus cement. A much higher cement stiffness of 8GPa resulted in larger changes in the stresses. The most significant parameter in this study was found to be the changed load path as a result of partial height restoration. This induced a moment in the construct and increased the stresses and strains in the anterior compartments of each vertebra as well as marked in the adjacent (upper and lower) vertebrae. The factor of safety calculation showed the centre of the L3 vertebra to be the most failure prone in all cases, with the osteoporotic bone models showing higher fracture tendencies.

This study indicates that healthier bone has a better chance of survival. Cement properties with lower cement elastic moduli induce stresses/strains which are more similar to the intact model. The best way to reduce the likelihood of failure is to restore the vertebral height.

Squeaking in ceramic on ceramic bearing total hip arthroplasty is well documented but its aetiology is poorly understood. In this study we have undertaken an acoustic analysis of the squeaking sound recorded from 31 ceramic on ceramic bearing hips. The frequencies of these sounds were compared with in vitro acoustic analysis of the component parts of the total hip implant. Analysis of the sounds produced by squeaking hip replacements and comparison of the frequencies of these sounds with the natural frequency of the component parts of the hip replacements indicates that the squeaking sound is due to a friction driven forced vibration resulting in resonance of one or both of the metal components of the implant. Finite element analysis of edge loading of the prostheses shows that there is a stiffness incompatibility between the acetabular shell and the liner.

The shell tends to deform, uncoupling the shell-liner taper system. As a result the liner tends to tilt out of the acetabular shell and slide against the acetabular shell adjacent to the applied load. The amount of sliding varied from 4–40μm. In vitro acoustic and finite element analysis of the component parts of a total hip replacement compared with in vivo acoustic analysis of squeaking hips indicate that either the acetabular shell or the femoral stem can act as an “oscillator’ in a forced vibration system and thus emit a squeak.

Introduction: Squeaking has long been recognized as a complication in hip arthroplasty. It was first reported in the Judet acrylic hemiarthroplasty.¹ It was the squeak of a Judet prosthesis that led John Charnley to investigate friction and lubrication of normal and artificial joints which ultimately led to the concept of low friction arthroplasty. Ceramic on ceramic bearings were pioneered by Boutin in France during the 1970’s, but experienced unacceptably high fracture rates. Charnley demonstrated in vitro squeaking when he tested one of Boutin’s ceramic-on-ceramic bearings in his pendulum friction comparator.² Squeaking has also been reported in other hard on hard bearings, and can also occur after polyethylene bearing surface failure resulting in articulation between metal on metal or ceramic on metal surfaces.^3–6 Recently, squeaking has been increasingly reported in modern ceramic-on-ceramic bearings in hip arthroplasty. However, although well-documented, the aetiology of squeaking in ceramic on ceramic bearings is still poorly understood. The incidence ranges from under 1% to 10%.^7–10 It has been reported in mismatched ceramic couples,11and after ceramic liner fracture.^12,13 An increased risk of squeaking has been demonstrated with acetabular component malposition, as well as in younger, heavier and taller patients.⁹ However, it may also occur in properly matched ceramic bearings with ideal acetabular component position and in the absence of neck to rim impingement.^7–9 In rare cases, the squeak is not tolerated by the patient and has prompted a revision.

Under ideal conditions hard-on-hard bearings are assumed to be operating under conditions of fluid film lubrication with very low friction.^14,15 However, if fluid film lubrication breaks down leading to dry sliding contact there will be a dramatic increase in friction. If this increased friction provides more energy to the system than it can dissipate, instabilities may develop in the form of friction induced vibrations and sound radiation¹⁶. Friction induced vibrations are a special case of forced vibration, where the frequency of the resulting vibration is determined by the natural frequency of the component parts. Running a moistened finger around the rim of a wine glass is an example of this. [Appendix].

The hypothesis of this study is that the squeaking sound that occurs in ceramic on ceramic hip replacement is the result of a forced vibration. This forced vibration can be broken down into a driving force and a resultant dynamic response¹⁷. The driving force is a frictional driving force and occurs when there is a loss of fluid film lubrication resulting in a high friction force^14,15,18. The dynamic response is a vibration of a part of the device (the oscillator) at a frequency that is influenced by the natural frequency of the part¹⁶. By analyzing the frequencies of the sound produced by squeaking hip replacements and comparing them to the natural frequency of the component parts of a hip replacement this study aims to determine which part produces the sound.

Materials and methods: In vitro determination of the natural frequencies of implant components Modal analysis has suggested that resonance of the ceramic components would occur only at frequencies above the human audible range and that resonance of the metal parts would occur at frequencies within the human audible range. Furthermore, that resonance of the combined ceramic insert and titanium shell would not be within the human audible range. To test this hypothesis we performed a simple acoustic analysis. The natural frequency of hip replacement components was determined experimentally using an impulse-excitation method (Grindo-sonic). Components were placed on a soft foam mat in a quiet environment and struck with a wooden mallet. The sound emitted from the component was recorded on a personal computer with an external microphone with a frequency response which ranges from 50Hz to 18,000Hz (Beyerdynamic MCE87, Heilbronn, Ger-many). The computer has an integrated sound card with a frequency response from 20Hz to 24kHz (SoundMAX integrated digital audio chip, Analogue Devices Inc, Norwood, M.A.) and we used a codec with a frequency response from 20Hz to 20kHz (Audio Codec ’97, Intel, Santa Clara, CA). Sound files were captured as 16 bit mono files at a sample rate of 48000Hz using acoustic analysis software (Adobe Audition 1.5, Adobe Systems Incorporated, San Jose, California, USA). We performed fast Fourier transform (FFT) of the sound using FFT size 1024 with a Blackmann-Harris window to detect the frequency components of the emitted sound. (Fast Fourier transform is an accepted and efficient algorithm which enables construction of a frequency spectrum of digitized sound).

We tested the following components: modular ceramic/titanium acetabular components, which included testing the titanium shell and the respective ceramic inserts both assembled according to the manufacturer’s instructions and unassembled; titanium femoral stems and ceramic femoral heads both assembled and unassembled. A range of sizes of each component was tested according to availability from our retrieval collection.

In vivo acoustic analysis: Sound recordings were collected from 31 patients. Nineteen recordings were made at our institution: 16 of these were video and audio recordings and 3 were audio only recordings. Video recording was with a digital video camera recorder (Sony DCR-DVD101E Sony Electronics, San Diego, CA, USA) with the same external microphone used in the in vitro analysis. For 3 patients who could not reproduce the sound in the office we lent them a digital sound recorder for them to take home and record the sound when it occurred (Sony ICD-MX20, Sony Electronics, San Diego, CA, USA). This device has a In vivo acoustic frequency range from 60Hz to 13,500Hz. The remainder of the recordings were video and audio recordings made by surgeons at three other institutions on digital video camera recorders.

Sound files were captured and analyzed by the same method used in the in vitro analysis. Each recording was previewed in the spectral view mode which allows easy visual identification of the squeak in the sound recording. In addition all sound recordings were played, listening for the squeak. Once a squeak was identified a fast Fourier transform (FFT) was performed. We used FFT size 1024 with a Blackmann-Harris window which allowed us to easily pick out the major frequency components. All prominent frequency components were recorded at the beginning of the squeak and at several time points during the squeak if there was any change. A range was recorded for the fundamental frequency component. We were able to determine the frequency range of the recording device used by observing the frequency range of the background noise on the recording. We found that if a squeak was audible on the recording we had no difficulty determining its frequency regardless of the quality of the device used to make the recording or the amount of background noise.

The mean age of the patients was 54 years (23 to 79 years), mean height was 171cm (152 to 186cm) and mean weight was 79kg (52 to 111kg). There were 17 female and 14 male patients. There were nineteen ABGII stem and ABGII cup combinations, 10 accolade stem and trident cup, 1 Exeter stem and trident cup and 1 Osteonics Securfit stem with an Osteonics cup. Ethics committee approval was obtained for this project from our institution and from the referring institutions and informed consent was gained from the patients.

Finite element analysis of edge loading: Edge-loading wear which may provide a mechanism for failure of fluid film lubrication and may therefore play a role in squeaking. To evaluate edge loading further we conducted finite-element analysis (FEA).⁹ Computed tomography (CT) scans of an intact pelvis were obtained from visual human data set (VHD, NLM, Bethesda, Maryland). Slices were taken at 1mm thick with no inter-slice distance through the entire pelvis. The CT files were then read into a contour extraction program and saved into an IGES file format which was imported into PATRAN (MSC Software, Los Angeles, CA) to develop the pelvic geometry. The pelvis was meshed with 10 noded modified tetrahedral elements. The model was reconstructed with a 54mm titanium alloy generic acetabular shell and a 28mm alumina ceramic liner. The acetabular shell and ceramic liner were meshed using 8 noded hexahedral elements. The shell-liner modular taper junction incorporated an 18° angle. The implant contact conditions (Lagrangian multiplier) allowed the liner and shell to slide with a friction coefficient of 0.9. Tied contact conditions were applied between the generic acetabular shell and the bone representing bone ongrowth. Bone material properties were extracted from the CT files by taking the Hounsfield value and the coordinates and mapping to the element in the model allowing us to calculate the Young’s modulus for each element ¹⁹. Material properties for the shell and liner were based on published values²⁰ for titanium alloy and alumina ceramic

Lacerations of the FDP tendon in zone one may be reattached to bone with a modified Bunnell pullout suture or with suture anchors. Eleven cadaveric fingers were submitted to cyclical testing of five hundred cycles with either a modified Bunnell pullout suture of 3-0 polypropylene or a micro-Mitek suture anchor with 3-0 Ethibond. Gap formation was 6.6mm in the modified Bunnell group and 2.0mm in the micro-Mitek group (p< 0.001). Load to failure was 37.6N in the pullout group and 28.5N in the anchor group (p< 0.005). Gap in the pullout group and low failure load in the anchor group are of concern.

Distal zone one FDP tendon lacerations are usually re-attached to bone by a modified Bunnell pullout suture of 3-0 polypropylene. This treatment may lead to moderate to severe losses of DIP joint motion in up to 50% of patients. Suture anchors have recently been introduced as a fixation alternative. Cyclical testing simulating five days of a passive mobilisation protocol was used to compare the Micro-Mitek anchor to the modified-Bunnell pullout suture in FDP tendon fixation.

Eleven cadaveric fingers FDP tendons were repaired to bone using a modified Bunnell pullout suture of 3-0 polypropylene or a micro-Mitek anchor with 3-0 Ethibond. Testing was done from 2N to 15N at 5N/sec, for a total of five hundred cycles. Gap formation at the tendon bone interface was measured. Load-to-failure was performed on all specimens.

No specimens failed during cyclic testing. Gap formation was 6.6mm (SD 1.2, range 4.9–8.2mm) and 2.0mm (SD = 0.4, range 1.7–2.7mm) for the pullout technique and the micro-Mitek anchor repair respectively (p< 0.001). Load to failure data was 37.6N (SD 4.7, range 31.8–45.1N) for the pullout group and 28.5N (SD 4.0, range 21.8–33.4N) for the micro-Mitek group (p< 0.005).

This data suggests that both fixation techniques may be adequate to sustain five days of simulated passive rehabilitation therapy. Significant gap formation in the modified Bunnell pullout group is of concern although this needs to be correlated in the clinical setting. The lower failure rate of the micro-Mitek group may leave a narrow margin of safety for passive rehabilitation.

The use of plates and screws for the treatment of certain metacarpal fractures is well established. Securing plates with bicortical screws has been considered an accepted practice. However, no study has questioned this.

This study biomechanically assessed the use of bicortical versus unicortical screws in metacarpal plating. Eighteen fresh frozen cadaveric metacarpals were subject to midshaft transverse osteotomies and randomly divided into two groups. Using dorsally applied Leibinger 2.3mm 4 hole plates, one group was secured using 6mm unicortical screws, while the second group had bicortical screws. Metacarpals were tested to failure using a four point bending protocol in an apex dorsal direction on a servo-hydraulic testing machine with a 1kN load cell. Load to failure, rigidity, and mechanism of failure were all assessed.

Each group had three samples that did not fail after a 900 N load was applied. Of those that failed, the mean load to failure was 596N and 541 N for the unicortical and bicortical groups respectively. These loads are well in excess of those experienced by the in-vivo metacarpal. The rigidity was 446N/mm and 458N/mm of the uni-cortical and bicortical groups respectively. Fracture at the screw/bone interface was the cause of failure in all that failed, with screw pullout not occurring in any.

This study suggests that there may be no biomechanical advantage in using bicortical screws when plating metacarpal fractures. Adopting a unicortical plating method simplifies the operation, and avoids potential complications associated with overdrilling and oversized screws.

The behaviour of two different methods of reattachment of the flexor digitorum profundus tendon insertion was assessed. Cyclical testing simulating the first 5 days of a passive mobilisation protocol was used to compare the micro Mitek anchor to the modified-Bunnell pull-out suture. Twelve fresh-frozen cadaveric fingers were dissected to the insertion of the FDP tendon. The FDP insertion was then sharply dissected from the distal phalanx and repaired using one of two methods: group 1 -modified Bunnell pullout suture using 3/0 Prolene; group 2 micro Mitek anchor loaded with 3/0 Ethibond inserted into the distal phalanx. Each repaired finger was mounted on to a material testing machine using pneumatic clamps. We cyclically tested the repair between 2N and 15N using a load control of 5N/s for a total of 500 cycles. Gap formation at the tendon bone interface was measured every 100 cycles.

No specimens failed during cyclical testing. After 500 cycles, gap formation of the tendon-bone interface was 6.6mm (SD = 1.2mm), and 2.1 mm (SD = 0.3mm) for the pullout technique and the micro Mitek anchor repair respectively. Concerns related to suture anchors, such as anchor failure or protrusion, joint penetration, and anchor-suture junction failure, were not encountered in this study.

Cyclical loading results suggest that the repair achieved with both methods of fixation is sufficient to avoid failure. However, significant gap formation at the tendon-bone interface in the modified Bunnell group is of concern, suggesting it may not be the ideal fixation method.

Clinical implantation represents the ultimate experiment of any component and often demonstrates areas of strengths and weaknesses not predicted from in vitro testing. Mobile bearing knees incorporate an additional articulating interface between the flat distal PE insert and a highly polished metal tibial tray. This can allow the proximal interface to retain high conformity whilst leading to reduced stresses at the bone – prosthesis interface by permitting complex distal interface compensatory motion to occur (rotation and/or translation). Retrieval reports on many of the new generation of mobile bearing implants remains scarce. This study presented a retrieval analysis of 9 mobile bearing inserts that had be in situ for less than 24 months.

Nine cemented mobile bearing implants (6 AP Glide, 1 LCS, 1 MBK and 1TRAK) were received into our Implant Retrieval Program. The femoral component, tibial tray and PE insert were macroscopically examined under a stereo-zoom microscope for evidence of damage. The PE inserts were graded for wear based on optical and SEM assessments. The proximal and distal surfaces of the PE inserts were subsequently assessed for surface roughness following ISO 97 (Ra and Rp) using a Surfanalyzer 5400 (Federal Products, Providence, RI). Virgin, unused PE inserts were analysed and served as a comparison to the retrieved implants.

Time in situ time for these implants ranged from 6 months to 24 months (mean 18.6). The implants were revised for instability and pain (AP glide) or dislocation (TRAK). Damage to the femoral components, in general, was minimal with some evidence of a transfer film of PE. The proximal surface of the tibial trays presented evidence of PE transfer as well as some scratches but in general were intact. The proximal PE and distal PE articulating surfaces demonstrated significant areas of damage due to third body wear which was identified on EDAX to be PMMA. Areas of burnishing were also present at the proximal and distal interface. The damage, in part, correlated with the complex kinematics of each design.

Introduction: The lordosis of the lumbar spine, flexion angle and body weight result in significant shear forces through the lumbar and lumbosacral disc spaces. These shear forces result in translational motion across the disc space, which is resisted but not completely abolished by pedicle screw stabilisation. Failure of lumbar interbody fusions through non-union may be related to translational micromotion at the vertebral endplate / bone graft interface. A porcine in vitro model was established to test whether variations in the design of inter-body implants and in particular, the presence of surface serrations would assist in resisting shear forces – especially those causing anterior translation.

Methods: Measurements of anterior vertebral translation were recorded on porcine cervical spine segments, subjected to 25 N antero-posterior shear load while under a 300 N compressive pre-load. Baseline testing was firstly performed on the intact specimens and following removal of the facet joints. The annulus, disc nucleus and cartilaginous endplates were then removed and the specimens were divided into two groups for testing using interbody implants. Four stainless steel blocks measuring 15 mm (length) × 5 mm (height) × 4 mm (width) were manufactured to act as intervertebral disc spacers. Two were made with smooth surfaces and two were made with 1 mm deep serrations on the upper and lower surfaces. One group was tested with two smooth and one with two serrated implants.

Results: Under 25 N shear load, the specimens tested with the serrated implants showed anterior vertebral translation of 0.046 ± 0.013 mm while those tested with the smooth surfaced implants measured 0.152 ± 0.075 mm (p < 0.01). A significant difference was also found between the stiffness of the specimens implanted with smooth surfaced (432.8 N/mm) and serrated (1088.4 N/mm) implants (p < 0.01). The value for peak load at failure for the specimens with smooth surfaced implants (150.43N) was less than those implanted with serrated implants (175.48 N), but not significantly different.

Discussion: The presence of surface serrations on the interbody implants significantly increased the resistance to shear forces in this model. In the clinical setting, we postulate that the degree of micromotion generated by anterior shear forces at interbody fusion sites should be substantially less when serrated implants are used and reduce the incidence of non-union. This may explain the improved fusion rates reported by contemporary authors when using some interbody implants. Further research is needed to clarify the combined effects of pedicle screw stabilisation and interbody implants upon shear displacement and variations in implant design.