To systematically review the literature regarding post-surgical treatment regimens on ankle fractures, specifically whether there is a benefit to early weightbearing or early mobilization (6 weeks form surgery).

The PubMed, MEDLINE and Embase databases were searched from inception to May 24, 2020. All randomized controlled trials that analyzed the effects of early weightbearing and mobilization following an ankle surgery were included. The primary outcome measure was the Olerud Molander Ankle Score (OMAS). Secondary outcomes included return to work (RTW) and complications. Logistic regression models with random intercepts were used to pool complication data by protocol clustered by study.

Twelve RCT's were included, with a total of 1177 patients (41.8 ± 8.4 years). In total, 413 patients underwent early weightbearing and early mobilization (35%), 338 patients underwent early weightbearing and delayed mobilization (29%), 287 patients underwent delayed weightbearing and early mobilization (24%), and 139 patients underwent delayed weightbearing and delayed mobilization (12%). In total, 81 patients had a complication (7%), including 53 wound complications (5%), 11 deep vein thromboses (1%), and 2 failures/nonunions (0%). Early ankle mobilization resulted in statistically significant increases in OMAS scores compared to delayed mobilization (3 studies [222 patients], 12.65; 95% CI, 7.07-18.22; P < 0.00001, I2 = 49%). No significant differences were found between early and delayed weightbearing at a minimum of one-year follow-up (3 studies [377 patients], 1.91; 95% CI, −0.73-4.55, P = 0.16, I2 = 0%). Patients treated with early weightbearing and early mobilization were at higher odds of facing any complication (OR 3.6, 95%CI 1.05-12.1, p=0.041) or wound complications (OR 4.9, 95%CI 1.3-18.8, p=0.022) compared to those with delayed weightbearing and delayed mobilization.

Early mobilization following surgical treatment for an ankle fracture resulted in improved ankle function scores compared to delayed mobilization regimens. There were no significant differences between early and delayed weightbearing with respect to patient reported outcomes. Patients who were treated with early mobilization and early weightbearing had an increased odds of postoperative complications.

The treatment of acute full thickness chondral damage within the knee is a surgical challenge. Frequently used surgical techniques include chondroplasty, micro-fracture and chondrocyte implantation. These procedures give unpredictable functional outcomes and if the formation of neocartilage is achieved it is predominantly composed of type 1 collagen.

The TruFit osteochondral plug was designed to provide a scaffold for cell proliferation into full thickness chondral defects. It is a composite polymer composed of polylactide co-glycolide, calcium sulphate and poly-glycolide fibres. It is composed of 2 layers, one with a similar trabecular network to cancellous bone and a superficial layer designed to simulate articular lining.

The TruFit bone plug was analysed using micro-computed tomography. Its morphology characteristics, granulometry, mechanical performance and image guided failure were tested as well as numerical modelling to assess the permeability of TruFit.

Morphological parameters of the TruFit bone plug compared favourably with those of human tissue. Under load the scaffold exhibited shear bands throughout the composite leading to a failure mechanism similar to cancellous bone. Stress relaxation rates of the scaffolds were greatly decreased under wet conditions, likely due to plasticisation of the scaffold by water.

The biomechanical properties of the TruFit bone plugs are a cause for concern. The Scaffolds mechanical performance under load rapidly deteriorates in wet conditions at body temperature (the natural knee environment). This early failure will lead to defects in the articular surface where the plug has been inserted. Clinical data is sparse. This study correlates with work performed by Dockery et al & Spalding et al. These clinical studies have shown that the TruFit implant shows no evidence of bone ingrowth or osteoconductivity. It provides no subchondral support to neocartilage or tissue that was stimulated to form around the defects and surgical sites.

Introduction

Novel hydrogel implants, TRUFIT® bone plugs, have been developed by Smith & Nephew to replace worn-out cartilage surfaces, restoring mobility and relieving joint pain. There is limited information, however, on the biomechanical properties of the implants. Therefore, appropriate mechanical testing and modelling must be carried out to assess their mechanical properties for load bearing applications.

In this study, compressive properties of TRUFIT® bone and dual layer implants were examined under selected physiological loading conditions. The bone layer of the implant was also modelled using a biphasic poroviscoelastic (BPVE) material constitutive law and the results from the model are compared with those from the experiments.

Introduction

Damage development in cemented acetabular replacements has been studied in bovine pelvic bones under long-term physiological¹ loading, albeit dry, conditions, using a specially designed hip simulator². In this work we report further experimental results from testing in wet condition in a new custom designed environmental chamber. Damage was detected and monitored using mCT scanning at regular intervals of the experiments. Two dimensional projections in the axial, sagittal and coronal planes were extracted from the 3D data for fatigue damage identification. The simulated mechanical and biological effects on the initiation and evolution of the damage of cemented acetabular reconstructs were examined and compared with those under dry condition.

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.

The long-term stability of total hip replacements (THRs) critically depends on the lasting integrity of the bond between the implant and the bone. Late failure in the absence of infection is known as ‘aseptic loosening’, a process characterised by the formation and progressive thickening of a continuous layer of fibrous tissue at the interface between the prosthesis and the bone. Aseptic loosening has been identified as the most common cause for long-term instability leading to the failure of ace-tabular cups. There is clearly a need to study the failure mechanisms in the acetabular fixation if the long-term stability of THR is to be significantly improved. The bonding strength in the presence of defects is measured using interfacial fracture toughness, and this information is not available currently.

In this work, interfacial fracture toughness of synthetic and bovine bone-cement interface has been studied using sandwiched Brazilian disk specimens. Experiments were carried out using a common bone cement, CMW, and polyurethane foam under selected loading angles from 0 to 25 degrees to achieve full loading conditions from tensile (mode I) to shear (mode II). Finite element analyses were carried out to obtain the solutions for strain energy release rate at a given phase angle (ratio of shear and tensile stress) associated with the experimental models. The effects of crack length on the measured interfacial fracture toughness were examined. Microscopic studies were also carried out to obtain the morphology of the fractured interfaces at selected loading angles.

The results show that both polyurethane foam and bovine cancellous bone seem to produce a similar type of interfacial failure of bone-cement interface, with cement pedicles being ‘pull-out’ of the pores of the foam/ bone. Damage sustained by the cement pedicles seems to increase progressively as the increase of shear loading component. The measured values of fracture toughness are a function of crack length and phase angle, and are comparable with those published in the literature on cortical bone and cement interface.

The implication of these results on the assessment of fixation in acetabular replacements is discussed, particularly in the light of results from bovine cancellous bone-cement interface.

Multiple biological and mechanical factors may be responsible for the failure of fixation in cemented total hip replacements (THRs). Although the eventual failure of THRs may appear to be biological, the initiation of the failure during early period post operation may well be mechanical. It is in this area that mechanistic analysis is of particular significance.

This study builds on work by Rapperport et al, Dals-tra and Huiskes on stress analysis of implanted acetabulum, while focuses on fracture mechanics analyses of fracture of cement and of bone-cement interface. Specifically, finite element models were developed where cracks of most favourable orientations in the cement mantle were simulated. Possible crack path selections were explored. A simplified multilayer experimental model was also developed to represent the implanted acetabulum, and fatigue tests were carried out on the model. The experimental results were compared with those from the FE model.

Furthermore, interfacial crack growth at bone-cement interface was simulated from the superior edge of the acetabulum, as suggested from the clinical observations. The strain energy release rates were computed for typical hip contact forces during gait and as a function of crack length. Associated phase angles were also computed to account for the materials mismatch. The results were evaluated against the interfacial fracture toughness of the bone-cement interface, measured using sandwich Brazilian disk specimens. The results show that although interfacial fracture seems to be unlikely for large phase angles where shear component is most active, the strain energy release rates are comparable with the values of the interfacial fracture toughness when mode I is predominant, suggesting interfacial fracture.

The study also shows that the fracture toughness of cement is much higher than the interfacial fracture toughness of bone-cement, this may explain the reason why interfacial fracture is favoured even if the crack driving force at bone-cement interface appears to be weaker than that in the cement mantle.

Objective: To examine the effect of varying the thickness of the cement mantle on the strain distribution near the bone-cement interface.

Background: An insufficient cement mantle is thought to generate cement fractures near the bone-cement interface. Debonding at the bone-cement interface may accompany such fractures, and, mechanical failure of the prosthesis may follow. In this study, we aim to analyse the relationship between the cement mantle thickness and the acetabular strain distribution near the bone-cement interface.

Experimental model: Four hemi-pelvic saw bones specimens were implanted with six protected precision strain gauges. All specimens were prepared to receive a 53/28 cemented polyethylene cup (Depuy Charnley Elite).

Methods: We simulated hip joint force relative to the cup during normal walking for quasi-static tests on an Instron 1603 testing machine. The magnitude of the maximum and minimum principal strains, and the orientation of the maximum principal strains were calculated based on the readings of strains from a 32 channel digital acquisition system.

Results: Statistically significant differences in the total strains per gait cycle (p< 0.001) have been noted at all gauge locations. In the principal load bearing quadrants, the recorded tensile strains are reduced by 50% as a result of the thicker mantle, while the transmission of compressive strain is enhanced.

Conclusion: A cement mantle thickness of 5-6mm may preserve the structural integrity of the principal load bearing quadrants of the acetabulum better than a mantle thickness of 2-3mm, by minimising the acetabu-lar strains. This maybe desirable in total hip replacements for conditions such as rheumatoid arthritis and osteoporosis, where the poorer quality bone can be assisted by recruitment of a larger surface area to participate in load bearing.

Keywords: Principal strains; Cement mantle; Mantle thickness; Bone-cement interface; Acetabular strains.

Retrieval studies based on revision operations at King Edwards VII Hospital reveal that, although micro-cracks develop in the cement mantle, it is the debonding between cement and bone that often defines the final failure of cemented acetabular replacements. This was illustrated at the revision surgeries by the easy removal of the acetabular cups with cement mostly attached to the cup. It is felt that a fundamental understanding of the mechanisms that initiate and propagate the interfacial failure at the bone-cement interface is the key towards solving the problem.

In this work, in-vitro fatigue tests were carried out on cemented acetabular replacements using third-generation of composite pelvic bones. Standard Charnley cups were implanted using common bone cement, CMW, following the standard surgical procedures. The implanted hemi-pelvic bone model was then constrained at the sacro-iliac and pubic joints to represent the anatomic constraint conditions. Cyclic loads representing the maximum range of the hip contact force during normal walking were used and the direction of the maximum hip contact force was achieved by using angled plates. In addition to standard cup position, open cup and retroverted cup positions were also examined to assess the significance of cup orientation under fatigue loading conditions.

Damage development in the reconstruction was monitored using CT scanning at regular intervals. Permanent records were collected and the sample was eventually sectioned and polished for microscopic studies. Results show excellent correlations between the results from the CT images and the microscopic studies, indicating progressive bone-cement interfacial failure in the posterior-superior quadrant.

The significance of the work in the studies of ‘aseptic loosening’ will be discussed.

A new hip simulator has been developed at the University of Portsmouth and manufactured at Simulation Solutions, Ltd. (UK) for the purpose of fatigue testing of implanted acetabula. Although hip simulators for in vitro wear testing of prosthetic materials in total hip arthroplasty (THA) have been available for many years, similar equipment has yet to appear for endurance testing of fixations in cemented THA, despite of considerable evidence of late aseptic loosening as one of the most singnificant failure mechanisms in acetabular replacements ^[1].

In this study, a new four-station hip simulator designed for in vitro fatigue testing of implanted acetabula is described. The four-station machine has spacious test cells that can accommodate full hemi-pelvic bones with implants. The machine was designed to simulate the direction and the magnitude of the hip contact force relative to the acetabular cup coordinate system, as reported by Bergmann et al. ^[2], under typical physiological loading conditions, including stair climbing as well as walking. The controls were designed as such that each station may operate independently with a loading waveform that is fully programmable. The motions were achieved through two encoded servomotors suitably connected to gearboxes; while the loading was realised through a close-looped pneumatic system. The motions and the resultant hip contact force of the new hip simulator were evaluated, and found to be satisfactory in reproducing the typical physiological loading waveforms including normal walking, ascending and descending stairs.

Experiments have been carried out using third generation composite bones (Pacific Research Laboratories, Inc.) and bovine bones. Both hip simulator and conventional fatigue testing were carried out. The implanted acetabula were CT scanned periodically to monitor the damage development in the fixation. Preliminary results seem to suggest that both magnitude and direction of the hip contact force influence the integrity of the fixa-tion, and failures appear to occur earlier in samples tested using the hip simulator. The predominant failure mechanism appears to be interfacial fracture, consistent with clinical observation of radiolucent lines and bone-cement interfacial failure.

Interfacial stress distributions in the acetabular region have been studied using plane strain finite element models before and after total hip replacement. The model was adapted from a roentgenogram of a 4 mm slice normal to the acetabulum through the pubic and ilium. The model was divided into 24 regions of different elastic constants with isotropic material properties assumed in each region. The femoral head was modelled as a spherical surface that was mated with a congruent spherical acetabular socket. The implanted hip model was developed by modifying the natural hip model. Contact analyses were carried out between the articulating cartilage layers and between a cobalt chromium head and a cemented ultra-high molecular weight polyethylene (UHMWPE) cup under selected hip contact load cases during normal walking. Local polar coordinates were employed to facilitate the calculation of the interfacial stress components between the cup and cement, cement and subchondral bone as well as between the subchondral and underlying cancellous bones.

The results show that severe reductions in the local stresses in subchondral and cancellous bones were found in the reconstructed case. Both the peak stress and the range of the stress were reduced substantially, suggesting stress shielding in the acetabular region. Load transfer in the reconstructed case was found to occur primarily in the cement layer superior to the cup. Both the peak stress and the stress variation in the cement mantle are substantial, whilst abrupt changes in interfacial stresses occurred between the cement and cup, and cement and subchondral bone. The influence of subchondral bone retention and thickness of the cement (up to 6 mm) on the interfacial stress distribution appears to be insignificant.

The work represents the first stage of research towards developing a numerical tool for pre/post operative assessment of cement/cementless acetabular components.

Background: It is thought that the forces transmitted across the hip joint produce migration of the prosthesis by failure at either the bone-cement or the prosthesis-cement interface. As symptoms associated with such motions often result from failure at the cement-bone interface, it is this interface and its sub-surfaces that are the critical areas of prosthesis loosening. Our aim is to produce a new and more accurate method of measuring strains at this critical interface.

Objective: To develop in-vitro experiments to measure the strain distributions near the bone-cement interface of the acetabulum region under physiological, quasi-static loading conditions.

Experimental Model: Two hemi-pelvic specimens of saw bones were used. Following careful placement of six protected precision strain gauges (4.6 x 6.4mm, tri-axial EA-13-031RB-120/E). One specimen was prepared to receive a cemented polyethylene cup (Depuy Charnley Ogee LPW 53/22). An uncemented 58mm Duraloc cup was implanted into a second specimen.

Methods: Hip joint force relative to the cup during normal walking (Bergmann, G., 2001. HIP98) was used for quasi-static tests on a Llody LR30K loading machine. The magnitude of the maximum and minimum principal strains, and the orientation of the maximum principal strains were calculated from a 32 channel digital acquisition system.

Results: For both specimens, the maximum principal strains at the maximum loading were highest in the medial wall (dome area) of the acetabulum. The tensile strain from the dome of the uncemented specimen at the maximum loading was twice that of the cemented specimen. In the cemented specimen, the compressive strains in the medial wall were almost twice the tensile strains at the maximum load. Within the acetabular quadrants, the highest strains were recorded in the posterio-inferior quadrant. Compressive strains in the posterio-inferior wall of the acetabulum seem to be comparable to those in the anterior-superior wall.

Conclusion: The critical areas for load transfer in the acetabulum are the medial wall (dome area), the posterio-inferior and the anterior-superior quadrants. The uncemented cup appears to provide a better load transfer mechanism than the cemented cup.

Objective: To develop in-vitro experiments that measure the strain distributions at the bone-implant and bone-cement interface of the acetabular region under physiological loading conditions for cemented and cementless sockets.

Experimental model: Four hemi-pelvic specimens of saw bones were used. Following careful placement of six protected precision strain gauges, two specimens were prepared to receive a cemented polyethylene cup (Depuy Charnley Elite 53/28). Another two specimens were prepared and implanted with un-cemented Duraloc 58/28 cups. Press-fit technique was validated by torque measurements.

Background: Symptoms associated with prosthetic migration result from osteoclast induced bone resorption at the interface adjacent to bone. We aim to develop a new and more accurate method of measuring strains at this critical interface.

Methods: To simulate quasi-static loading, selected variables of hip joint force relative to the cup during normal walking was used for quasi-static tests on an Instron 1603 testing machine. The magnitude and orientation of the principal strains (maximum and minimum) were calculated based on the readings of strains from a 32 channel digital acquisition system.

Results: The magnitude and distribution of acetabular trabecular bone strains are dependent on the type of cup material (un-cemented/cemented) implanted.

At the position of maximum load, the maximum principal strain in the un-cemented specimens was 14.4 times higher than that for the cemented specimens (T-value = −96.40, P-value = 0.007). The highest recorded tensile strains in these specimens were localised to the acetabular rim of the posterior-superior quadrant.

For the cemented specimens, the maximum principal strains are highest in the dorsal acetabulum, at a location that approximates to the centre of rotation of the replaced hip joint.

Shear strains in the posterior-superior quadrant of both cementless and cemented acetabuli surpass the maximum principal strains.

Conclusion: In both cemented and un-cemented specimens, the maximum shear and principal strains magnitude show similar spatial and statistical distribution. As indicators of local failure prospect within the acetabulum, these strains suggest that the posterior-superior quadrant is the most likely site for load-induced micro-fractures, in both cemented and cementless acetabuli.