Objectives. There remains conflicting evidence regarding cortical bone strength following bisphosphonate therapy. As part of a study to assess the effects of bisphosphonate treatment on the healing of rat tibial fractures, the mechanical properties and radiological density of the uninjured contralateral tibia was assessed. Methods. Skeletally mature aged rats were used. A total of 14 rats received 1µg/kg ibandronate (iban) daily and 17 rats received 1 ml 0.9% sodium chloride (control) daily. Stress at failure and toughness of the tibial diaphysis were calculated following four-point bending tests. Results. Uninjured cortical bone in the iban group had a significantly greater mean (standard deviation (. sd. )), p < 0.001, stress at failure of 219.2 MPa (. sd. 45.99) compared with the control group (169.46 MPa (. sd. 43.32)) following only nine weeks of therapy. Despite this, the cortical bone toughness and work to failure was similar. There was no significant difference in radiological density or physical dimensions of the cortical bone. Conclusions. Iban therapy increases the stress at failure of uninjured cortical bone. This has relevance when normalising the strength of repair in a limb when comparing it with the unfractured limb. However, the 20% increase in stress at failure with iban therapy needs to be interpreted with caution as there was no corresponding increase in toughness or work to failure. Further research is required in this area, especially with the increasing clinical burden of low-energy diaphyseal femoral fractures following prolonged use of bisphosphonates. Cite this article: Bone Joint Res 2015;4:99–104

Re-rupture rates after rotator cuff repair remain high because of inadequate biological healing at the tendon-bone interface. Single-growth factor therapies to augment healing at the enthesis have so far yielded inconsistent results. An emerging approach is to combine multiple growth factors over a spatiotemporal distribution that mimics normal healing. We propose a novel combination treatment of insulin-like growth factor 1 (IGF-1), transforming growth factor β1 (TGF-β1) and parathyroid hormone (PTH) incorporated into a controlled-release tyraminated poly-vinyl-alcohol hydrogel to improve healing after rotator cuff repair. We aimed to evaluate this growth factor treatment in a rat chronic rotator cuff tear model. A total of 30 male Sprague-Dawley rats underwent unilateral supraspinatus tenotomy. Delayed rotator cuff repairs were then performed after 3 weeks, to allow tendon degeneration that resembles the human clinical scenario. Animals were randomly assigned to: [1] a control group with repair alone; or [2] a treatment group in which the hydrogel was applied at the repair site. All animals were euthanized 12 weeks after rotator cuff surgery and the explanted shoulders were analyzed for biomechanical strength and histological quality of healing at the repair site. In the treatment group had significantly higher stress at failure (73% improvement, P=0.003) and Young's modulus (56% improvement, P=0.028) compared to the control group. Histological assessment revealed improved healing with significantly higher overall histological scores (10.1 of 15 vs 6.55 of 15, P=0.032), and lower inflammation and vascularity. This novel combination growth factor treatment improved the quality of healing and strength of the repaired enthesis in a chronic rotator cuff tear model. Further optimization and tailoring of the growth factors hydrogel is required prior to consideration for clinical use in the treatment of rotator cuff tears. This novel treatment approach holds promise for improving biological healing of this clinically challenging problem

Healing after bone fracture is assessed by frequent radiographs, which expose patients to radiation and lacks behind biological healing. This study aimed to investigate whether the electrical impedance using electrical impedance spectroscopy correlated to quantitative scores of bone healing obtained from micro-CT and mechanical bending test. Eighteen rabbits were subjected to tibial fracture that was stabilized with external fixator. Two electrodes were positioned, one electrode placed within the medullary cavity and the other on the lateral cortex, both three millimeters from the fracture site. Impedance was measured daily across the fracture site at a frequency range of 5 Hz to 1 MHz. The animals were divided into three groups with different follow-up time: 1, 3 and 6 weeks for micro-CT (Bone volume/tissue volume (BV/TV, %)) and mechanical testing (maximum stress (MPa), failure energy (kJ/cm3), young modulus (Mpa)). There was a statistically significant correlation between last measured impedance at 5 Hz frequency immediately prior to euthanasia and BV/TV of callus (−0.68, 95%CI: (−0.87; −0.31)). Considering the mechanical testing with three-point bending, no significant correlation was found between last measured impedance at 5 Hz frequency immediately prior to euthanasia and maximum stress (−0.35, 95%CI: (−0.70; 0.14)), failure energy (−0.23, 95%CI: (−0.63; 0.26)), or young modulus (−0.28, 95%CI: (−0.66; 0.22)). The significant negative correlation between impedance and BV/TV might indicate that impedances correlate with the relative bone volume in the callus site. The lack of correlation between impedance and mechanical parameters when at the same time observing a correlation between impedance and days since operation (0-42 days), might indicate that the impedance can measure biological changes at an earlier time point than rough mechanical testing

Aim. To investigate the effects of strain rate and mineral level on the stress at failure, stiffness and toughness of whole bones. Methods. 40 ovine femurs were harvested and subjected to either slow [8.56 × 10−3 s−1 (± 1.42 × 10−3 SD)] or dynamic [17.14 s−1 (± 8.20 SD)] loading. Half the bones were demineralised by 20% compared to the original mineral content. These were allocated evenly between the high and low strain rate groups. Dynamic loading was achieved by custom designed comminution device. Slow rate testing was carried out on a Zwick/Roell z005 testing machine. Results. Strain rate was found to increase the Young's modulus in both the normal and demineralised bone. Additionally the toughness of the bones at failure was found to reduce with increasing strain rate. When comparing bone of normal quality the stress at failure was found to increase with strain rate. However, this effect was greatly reduced when comparing the effect of strain rate on the stress occurring in demineralised bones. Discussion. These results show that bone has an ability to withstand higher than normal stresses if these are applied quickly and for a short duration (such as would occur in a traumatic event). This ability is greatly reduced when the bone is of reduced mineral content, such as is found in aged or diseased bone

The effect of bisphosphonates on the mechanical properties of the uninjured contra-lateral cortical bone during fracture healing is poorly reported. There remains conflicting evidence with regards the effect of bisphosphonate therapy on cortical bone strength. We assessed the effect of nine weeks of Ibandronate therapy, in a dose known to preserve cancellous bone BMD and strength, on the mechanical properties of the uninjured rat tibial diaphyses using a standardised model of tibial osteotomy and plate fixation. Skeletally mature ex-breeder rats were used. Stress at failure of the tibial diaphyses was measured by a four-point bending test using a custom made jig for rat tibiae. The mechanical strength was compared with radiographic measurements of bone density. Animals received daily subcutaneous injections. 11 rats received 1μg/kg Ibandronate (IBAN) daily and 17 rats received 1ml 0.9% Sodium Chloride (CONTROL) daily. The IBAN group had a statistically significant, p=0.024, higher stress at failure 212.7 (±42.04) MPa compared to the CONTROL group 171.7 (±46.13)MPa. There was a positive correlation between the mechanical strength of bone and the radiological measure of bone density. Osteopenia is known to occur following a fracture even in the contra-lateral limb. This study demonstrates that ibandronate therapy has no detrimental effect and may even increase the strength of uninjured cortical bone during the fracture healing process. The longer term effect of ibandronate on cortical bone especially in relation to the accumulation of mico-damage requires further study. Bisphosphonate effect on the uninjured limb needs to be considered when reporting proportional strength of fracture repair compared to the uninjured limb

Aims

Mechanical stimulation is a key factor in the development and healing of tendon-bone insertion. Treadmill training is an important rehabilitation treatment. This study aims to investigate the benefits of treadmill training initiated on postoperative day 7 for tendon-bone insertion healing.

The aim of this study was to characterize the effect of ligament water content on the accumulation of damage in vitro. MCLs of the rabbit knee were subjected to a constant cyclic stress for twenty-four hours (isotonic or hypotonic solution) and then failed. Ligaments cycled in hypotonic solution at 0.1 Hz demonstrated significantly more cyclic strain during loading and had significant reductions in both failure stress and failure strain. This study has demonstrated that elevated tissue water content influences the accumulation of damage in ligaments subjected to repetitive loading in vitro, leading to reductions in both strength and failure strain. Ligament water content may be altered in vivo during activity, following injury or during surgical intervention; tissue hydration can also be manipulated during in vitro testing. Currently, the effect of ligament water content on the accumulation of damage with loading is unknown. The aim of this study was to characterize the effect of ligament water content on the accumulation of damage during in vitro mechanical testing. We hypothesized that Medial Collateral Ligaments (MCL) subjected to repetitive stress under hypotonic conditions would accumulate more damage than ligaments loaded in an isotonic environment. MCLs of the rabbit knee were subjected to a constant cyclic stress (28 MPa) in this ex vivo model of loading. Tissues were bathed in either an isotonic or hypotonic solution (10% or 0.1% Sucrose) and were cycled at one of two frequencies (1 Hz or 0.1 Hz) for 24 hours followed by failure testing. After twenty-four hours of loading, ligaments cycled in hypotonic solution at 0.1 Hz had statistically significant reductions in both failure stress and failure strain. This group also demonstrated significantly more cyclic strain during loading than MCLs cycled in isotonic solution. Surprisingly, a significant difference in cyclic modulus was not detected between groups. This study has demonstrated that elevated tissue water content influences the accumulation of damage in ligaments subjected to repetitive loading in vitro, leading to reductions in both strength and failure strain. The interaction between tissue water content, cyclic strain and tissue damage will be the focus of further study

Objectives. Small animal models of fracture repair primarily investigate indirect fracture healing via external callus formation. We present the first described rat model of direct fracture healing. Methods. A rat tibial osteotomy was created and fixed with compression plating similar to that used in patients. The procedure was evaluated in 15 cadaver rats and then in vivo in ten Sprague-Dawley rats. Controls had osteotomies stabilised with a uniaxial external fixator that used the same surgical approach and relied on the same number and diameter of screw holes in bone. Results. Fracture healing occurred without evidence of external callus on plain radiographs. At six weeks after fracture fixation, the mean stress at failure in a four-point bending test was 24.65 N/mm. 2. (. sd. 6.15). Histology revealed ‘cutting-cones’ traversing the fracture site. In controls where a uniaxial external fixator was used, bone healing occurred via external callus formation. Conclusions. A simple, reproducible model of direct fracture healing in rat tibia that mimics clinical practice has been developed for use in future studies of direct fracture healing

Introduction. Bending tests are commonly used to evaluate the mechanical behaviour of small animal bones. To test whole bones, it is normal that soft tissue should be removed before testing. However, cleaning the specimens might disturb the callus, interfering with the mechanical properties. This study compares mechanical properties of rat tibia between specimen with and without muscle cleaning. Materials and methods. 12 male Wistar rats aged 3–4 months were used. Soft tissues including skin and muscle were removed from right tibias (Group A), whereas muscles on the left tibia were left intact (Group B). 4-point bending was used to find the ultimate load, stress and Young's modulus. Results. All specimens displayed a basic failure pattern from load-displacement graph. Mean ultimate load of 105.2N (S.E.M.=7.60, n=12) for group A and 101.6N (S.E.M.=7.32, n=12) group B. Mean stress at failure of 281.50MPa (S.E.M.=34.98, n=12) was found for group A 288.70MPa (S.E.M.=20.83, n=12) for group B. Mean Young's modulus was 8.97GPa (S.E.M.=1.44, n=12) for group A and 10.05GPa (S.E.M=0.69, n=12) for group B. No statistical differences for any mechanical properties were found between groups for either t-test(p>0.05) or Bland-Altman plot. Discussion. No differences in mechanical properties were found with or without soft tissue. Therefore, biomechanical testing of small specimen bending without removing muscle can be used. The advantages of this technique are reduced specimen preparation time and decreasing risk of creating a stress raiser at the callus

Objectives. Opening wedge high tibial osteotomy (HTO) is an established surgical procedure for the treatment of early-stage knee arthritis. Other than infection, the majority of complications are related to mechanical factors – in particular, stimulation of healing at the osteotomy site. This study used finite element (FE) analysis to investigate the effect of plate design and bridging span on interfragmentary movement (IFM) and the influence of fracture healing on plate stress and potential failure. Materials and Methods. A 10° opening wedge HTO was created in a composite tibia. Imaging and strain gauge data were used to create and validate FE models. Models of an intact tibia and a tibia implanted with a custom HTO plate using two different bridging spans were validated against experimental data. Physiological muscle forces and different stages of osteotomy gap healing simulating up to six weeks postoperatively were then incorporated. Predictions of plate stress and IFM for the custom plate were compared against predictions for an industry standard plate (TomoFix). Results. For both plate types, long spans increased IFM but did not substantially alter peak plate stress. The custom plate increased axial and shear IFM values by up to 24% and 47%, respectively, compared with the TomoFix. In all cases, a callus stiffness of 528 MPa was required to reduce plate stress below the fatigue strength of titanium alloy. Conclusion. We demonstrate that larger bridging spans in opening wedge HTO increase IFM without substantially increasing plate stress. The results indicate, however, that callus healing is required to prevent fatigue failure. Cite this article: A. R. MacLeod, G. Serrancoli, B. J. Fregly, A. D. Toms, H. S. Gill. The effect of plate design, bridging span, and fracture healing on the performance of high tibial osteotomy plates: An experimental and finite element study. Bone Joint Res 2018;7:639–649. DOI: 10.1302/2046-3758.712.BJR-2018-0035.R1

During hip revision removal of old cement mantle is a major problem. In cases of satisfactory bond between cement mantle and the underlying bone, cementing the revision stem into the old mantle is regarded as a highly attractive option. The aim was the analysis of the shearing strength of the interface between two layers of poly-methylmethacrylate cement in the presence of fluid. A laboratory, two-dimensional model of the interface was used. Effect of different viscosity fluids and volumes on its strength was checked. 6 variants (control monoblock, dry surface, surface stained with small or large volume of water or highly viscous fluid) containing 7 repeats were exposed to a single shearing stress to failure. Large volume of viscous fluid prevented bonding completely in two cases and significantly weakened the other samples showing mean failure stress of 5.53 MPa. This was significantly lower compared with control monoblock (19.8MPa), dry surface variant (16.9MPa) and the stain with small amount high viscosity fluid (16.01MPa). Interestingly, presence of a large volume of low viscosity fluid did not significantly reduce resistance to shear stress (17.05MPa). In all but large volume of viscous fluid variants, the failure occurred away from the interface between two cement layers. Large amount of viscous fluid weakened significantly this interface. If such a viscous fluid could be eliminated by copious water irrigation it is likely that strength of the cement-cement bond will be maintained. Our observations suggest that cement-in-cement technique seems to be biomechanically acceptable

Fractures repair by two mechanisms; direct fracture healing and indirect fracture healing via callus formation. Research concerning the effects of bisphosphonate on fracture repair has solely assessed indirect fracture healing. Patients with osteoporosis on bisphosphonates continue to sustain fragility fractures. A proportion of osteoporotic fractures require plate fixation. Bisphosphonates impair osteoclast activity and therefore, may adversely affect direct fracture healing that predominates with plate fixation. Five skeletally mature Sprague-Dawley rats received daily subcutaneous injections of 1mg/kg Ibandronate (IBAN). Similarly, five control rats received saline (CONTROL). Three weeks following commencement of injections a tibial osteotomy was rigidly fixed with compression plating similar to that seen in routine clinical practice. Fracture healing was monitored with radiographs. Six weeks post plate fixation, animals were sacrificed. Radiographs were performed of the extricated tibiae following plate removal. The visibility of the osteotomy site was scored as totally visible, partially visible or absent as previously described. Mechanical testing was conducted on the healing osteotomies via 4-point bending. Fractures healed without visible external callus. In the IBAN group three animals had totally visible osteotomy lines and two had partially visible osteotomy lines. The CONTROL group had three animals with absent osteotomy lines and two with partially visible osteotomy lines. The mean (±SD) stress at failure for the healing tibial osteotomies at 6 weeks was 28.8 (±23.97)MPa in the IBAN group and 37.4(±29.20) MPa in the CONTROL group (p=0.62). Our results indicate that Ibandronate adversely affected direct fracture repair as demonstrated by the radiographic density of the fracture line. The strength of the repair was reduced but this did not reach statistical significance. Our results suggest that a sample size of 220 animals is required to detect a 15% difference (alpha 0.05, beta 0.2) which suggests the effect of bisphosphonates on direct fracture repair may be small

We performed a study to evaluate the material properties of a new cylindrical scaffold plug licensed for the treatment of osteochondral defects as prior to the removal of a core of normal femoral condylar bone, it is imperative that the biomechanical properties of replacement implant material are known. TruFit CB plugs (Smith and Nephew) are resorbable material composed of polylactide-co-glycolide (PLG) copolymer, calcium-sulfate, polyglycolide (PGA) fibres and surfactant. The implants are 7mm, 9mm and 11mm cylindrical plugs. The stress/strain relationships of both the dual layer implant and the base layer material were examined. Compressive load testing at selected strain rates was performed in both confined and unconfined models in a substitute body fluid filled chamber. Compressive failure was found to occur between 40–60% strain with maximum stresses at failure for the dual layer implants occurring at 5.5MPa (7mm), 5.8MPa (9mm) and at 8.5MPa (11mm). The mechanical strength under constrained loading conditions is higher than in unconstrained loading (compressive stress required to develop 5 percent strain being 0.6MPa unconfined to 1.1MPa confined for 7mm; 0.6MPa to 1.4MPa for 9mm and 1.0MPa to 3.2MPa for 11mm implants). This demonstrates the importance of a close press fit. The modulus of elasticity was calculated at 50 MPa (7mm), 60 MPa (9mm) and 80 MPa (11mm). The larger the plug size, the higher the strength shown under test conditions at all strain rates. Prior to this study, the material properties of this implant have not been characterized. The Young’s moduli of the implants are in keeping with previous estimated values for successful regeneration of cartilage within a synthetic scaffold. The biomechanical properties described in this study will help to guide surgeons in TruFit CB use and guide the rehabilitation programmes of those patients who have had osteochondral lesions treated with TruFit CB scaffold plugs

Purpose: Little has been written about the size of bone defect that could be restored with one-stage lengthening over a reamed intramedullary nail. The aim of this study was to investigate the mechanical properties of the callus created at gaps of various sizes in sheep tibiae treated with reamed intramedullary nailing. Material-Methods: Sixteen adult female sheep were divided into four main groups: a simple osteotomy group (group I) and three segmental defect groups (1, 2, and 3 cm gap; groups I to III). One intact left tibia from each group was also used as the non-osteotomized intact-control group (group V). In all cases the osteotomy was fixed with an interlocked Universal Humeral Nail after 7 mm reaming. The osteotomized site was closed in layers including the periosteum without additional bone grafting and the limb was protected with long soft cast for 5 weeks postoperatively. Healing of the osteotomies was evaluated after 16 weeks by biomechanical testing. The examined parameters were torsional stiffness, shear stress and angle of torsion at the time of fracture. Results: Samples with a simple osteotomy or 1 cm gap were fractured distally to the callus zone, whereas samples with 2 and 3 cm gaps were fractured at the callus zone or at distal metaphysis. The regenerate bone obvious in the x-rays in the group of 1 cm and 2 cm gap had considerable mechanical properties. Torsional stiffness in these two groups was nearly similar and its value was about 60% of the stiffness of the simple osteotomy group. A gradually decreased stiffness was observed as the osteotomy gap increased. There was a decrease in maximum shear stress from simple osteotomy to osteotomy with a fracture gap of 3 cm. No significant differences were found among the angles of torsion at fracture for the various osteotomies or the intact bone. Our results showed that the group of 1 cm gap had the 65% of the shear stress at failure of the simple osteotomy group. Conclusion: We believe that there is evidence indicating that intramedullary nailing would be a reasonable option when one-stage lengthening of a long bone of 1 or 2 cm is contemplated

Introduction: Weber B fractures are one of the most common fractures of the ankle. Unstable fractures are treated with lateral plating and a lag screw. Another method of fixation is antiglide plating, this concept was first introduced by Brunner and Weber in 1982. Manoli and Schaeffer in 1987, showed that fixation by antiglide plate demonstrated superior static biomechanical properties compared to lateral plating. However there are some shortcomings in their study and hence we decided to perform our biomechanical study. The shortcomings of the Manoli study are. They did not use an interfragmentary lag screw for lateral plate fixation. It was a cadaveric study where the bone does not accurately represent the live bone. The quality of the bone ranging from normal to osteoporotic bone varies from cadaver to cadaver and hence there is no uniformity between the samples. Materials and Methods: We used 4th generation composite bone models validated to closely simulate human bone characteristics for fracture toughness, tensile strength, compressive strength, fatigue crack resistance and implant subsidence. 4th generation composite bone model provides uniformity of test samples which is not achievable in cadaveric studies. These bones were custom made for the experiment. We used two sets of bones, one representative of normal bone (Set A n=10) and the other of osteoporotic bone quality (Set B n=10). Each of the sets A & B will have two types of fixations for artificially created Weber B Fractures. Lateral plate with interfragmentary lag screw. Antiglide plate with interfragmentary lag screw. The strength of the fixation was measured by restressing the bone until the fixation failed using an Instron machine which simultaneously applied torque and compressive forces to the fibular construct. The resulting data was analysed on a computer and statistical analysis was performed. Results: When the two fixation constructs were stressed to failure, the lateral plate construct demonstrated less stiffness (3–5Nm/degree) and failed at lower energy levels (250Nm). Similar values obtained for the antiglide system were, stiffness of 12–16Nm/degree and energy absorbed to failure 350–450Nm. Antiglide plating was significantly more stable in the osteoporotic fibula. Conclusion: Antiglide plating with lag screw is much more stable than lateral plating. It is suitable for treatment without plaster cast post operatively. It results in a more stable fixation in osteoporotic bone

Background: During cemented hip arthroplasty revision removal of all the old cement mantle is a time staking process with multiple disadvantages. In some selected patients cementing revision stem into the old mantle is regarded as a highly attractive option. Contradictory evidence exists whether bond between two cement layers is strong enough, especially in the presence of interfering fluids. Aim: analysis of the shearing strength of the interface between two layers of polymethylmethacrylate cement in the presence of fluid. Methods: Cylindrical blocks of polymethylmethacrylate cement represented primary cement mantle. Its flat surface was machined to reproduce smooth old cement mantle surface comparable with that after removal of a highly polished stem (Ra=200nm). A second block was cast against the first and their junction represented the investigated interface. The influence of fluid was examined by injecting liquid onto the ‘primary’ surface prior to casting. Water or 2% water solution of carboxy-methyllcellulose (representing bone marrow viscosity of 400mPas) were used in two volumes: 0.02ml/cm2 (small) or 0.4ml/cm2 (large - surface submerged). 6 variants (control monoblock, dry surface, surface stained with small or large volume of water or highly viscous fluid) containing 7 repeats were exposed to a single shearing stress to failure at the speed of 1mm/min (Autograph AGS, Shimadzu, Japan). Results were analyzed using 1-way ANOVA with post-hoc analysis (equal N HSD) and power calculations. Results: Large volume of viscous fluid prevented bonding completely in two cases and significantly weakened the other samples showing mean failure stress of 5.53 MPa (95%CI:1.33–9.73 MPa). This was significantly lower compared with control monoblock (19.8–95% CI: 17.8–21.9 MPa), dry surface variant (16.9–95% CI: 15.9–18.0 MPa) and that stained with small amount of high viscosity fluid (16.01–95% CI: 15.12–17.0 MPa). Interestingly, presence of a large volume of low viscosity fluid (water) did not significantly reduce resistance to shear stress (17.05 – 95% CI:15.67–18.43 MPa). Similar relations were observed when strain at failure and toughness were analyzed. Conclusions: In all but large volume of viscous fluid variants, the failure occurred away from the interface between two cement layers. Large amount of viscous fluid weakened significantly this interface. If such a viscous fluid can be eliminated by copious water irrigation it is likely that strength of the cement-cement bond will be maintained. In the presence of low viscosity fluids (water, blood) careful use of gun technique is likely to allow for their escape as the cement is advanced within the femoral or the old mantle canal leading to a satisfactory bond. Our observations suggest that cement-in-cement technique seems to be biomechanically acceptable

Vast amount of literature is available on mechanical properties of PMMA, but not about the composite specimens of old and new cement. This is important, as in cement revision has become established technique with good clinical results. Originally Greenwald and later Li described properties of such specimens. However in these studies the old samples were only few days old, unlike clinical situation, where the old cement is a few years old. We therefore decided to test short-term mechanical properties of composite specimens and compare these with new uniform specimens. We choose specimens of cement 3–17 years old (median 11.8) for the manufacturing of the composite specimens. Material and Methods: Uniform and composite specimens were fabricated and were tested for bending, tensile and shear strength. Beam shaped specimens were fabricated for bending and tensile tests, cylindrical for shear. Seventeen beams and eight cylindrical specimens fabricated earlier (1988–2002) using the same moulds were available to form composite specimens. Old specimens were placed into the moulds and new cement was injected next to these. Specimens were allowed to polymerize at room temperature for 30 minutes and stored in saline at 37 °C for 6 weeks before testing. Specimens were tested in Lloyds EZ 20 machine with customized jig so that the junction was subjected to bending, tensile or shear force. Results: Bending tests: The load and bending stress for new specimen was 80N and 47MPa as compared with 72N and 38MPa for composite specimens. 4 composite specimens failed though old cement, 3 through the junction and 1 through the new cement. There was no statistical difference in maximum load between uniform and composite specimens (p=.29). However there was a difference in the stress between uniform and composite specimens. Tensile tests: The load and tensile stress for new specimen was 916N and 29MPa as compared with 795N and 24MPa for composite specimens. 7 composites failed through old cement, 1 through new cement and 1 at junction. There was difference in the load and stress of uniform specimens as compared with composite specimens. Shear tests: The load and shear stress for new specimen was 2718N and 35MPa as compared with 2055N and 26MPa for composite specimens. There was significant difference in load as well as stress in uniform specimens as compared with composite specimens. Discussion: This study demonstrates that composite specimens fail at 89.6% of bending load, 77.2% of tensile and 74.6% of shear load as compared with uniform new cement specimens. They have 81.4% of bending stress, 74.9% of tensile stress and 73.3% of shear stress at failure as compared with uniform specimens. Of more importance is the fact that only four of these composite specimens (23.5%) failed at the junction and the rest thirteen failed either through old cement (64.7%) or through new cement (11.8%)

We hypothesise that the stiffness of the acetabular component influences the stresses transmitted to bone. Thus stress shielding or stress overload of the underlying host bone may be influenced by the choice of fixation method. In addition, we believe that the so called “brake drum effect” plays a significant role in the development of rim stresses and subsequent failure of fixation. We have constructed a jig which allows the direct comparison, under controlled conditions, of contact stresses measured behind the acetabular component of polyethylene controls, uncemented metal backe cups and cemented all polyethylene cups, under physiological load. The design of the jig also allows measurement of stresses transmitted to the acetabular rim of the same three prostheses in order to confirm the presence and magnitude of the brake drum effect. The contact stresses are measured by miniature pressure transducers which are inserted through specially drilled holes in the shell of the jig so that the transducer is flush with the prosthesis under test. A total of 6 transducers are arranged in concentric circles radiating away from the prosthetic dome, so that contact stresses may be directly measured in various parts of the acetabular bed under conditions that reproduce as closely as possible the situation in a total hip prosthesis in vivo. Thus our method can be compared to other laboratory and theorectical techniques for investigation into stress transmission around acetabular components. The transducers were callibrated prior to each test to ensure parity of test results. The transducers were prestressed to ensure contact prior to each test. 6 polyethylene uncemented liners were tested alone in the jig to act as a control. In the same fashion, the same 6 polyethylene components were tested firstly in an uncemented, metal back acetabular component, and then as a cemented, all polyethylene component. The results indicate that significantly less stress is transmitted to bone when metal back components are used as compared to cemented components and controls. The data confirms that the brake drum effect occurs in both cemented and uncemented prostheses, leading to at least the absence of compressive forces at the prosthetic rim and in some circumstances tensile forces

Objectives

Rotator cuff tears are among the most frequent upper extremity injuries. Current treatment strategies do not address the poor quality of the muscle and tendon following chronic rotator cuff tears. Hypoxia-inducible factor-1 alpha (HIF-1α) is a transcription factor that activates many genes that are important in skeletal muscle regeneration. HIF-1α is inhibited under normal physiological conditions by the HIF prolyl 4-hydroxylases (PHDs). In this study, we used a pharmacological PHD inhibitor, GSK1120360A, to enhance the activity of HIF-1α following the repair of a chronic cuff tear, and measured muscle fibre contractility, fibrosis, gene expression, and enthesis mechanics.