Objectives

Previous studies have evidenced cement-in-cement techniques as reliable in revision arthroplasty. Commonly, the original cement mantle is reshaped, aiding accurate placement of the new stem. Ultrasonic devices selectively remove cement, preserve host bone, and have lower cortical perforation rates than other techniques. As far as the authors are aware, the impact of ultrasonic devices on final cement-in-cement bonds has not been investigated. This study assessed the impact of cement removal using the Orthosonics System for Cemented Arthroplasty Revision (OSCAR; Orthosonics) on final cement-in-cement bonds.

Aims

There are limited published data detailing the volumetric material loss from tapers of conventional metal-on-polyethylene (MoP) total hip arthroplasties (THAs). Our aim was to address this by comparing the taper wear rates measured in an explanted cohort of the widely used Exeter THA with those measured in a group of metal-on-metal (MoM) THAs.

Numerous studies have evidenced cement-in-cement techniques as reliable in revision arthroplasty. The original cement mantle is commonly reshaped to aid accurate placement of the new stem.

Ultrasonic devices selectively remove cement, preserve host bone and have lower cortical perforation rates than other techniques. As far as the authors are aware, their impact on final cement-cement bonds has not been investigated. This study assessed the impact of cement removal using OSCAR (Orthosonics System for Cemented Arthroplasty Revision, ORTHOSONICS) on final cement-cement bonds.

Twenty-four specimens were manufactured by pouring cement (Simplex P Bone Cement, Stryker) into stainless-steel moulds with a central rod polished to Stryker Exeter V40 specifications. After cement curing, rods were removed and eight specimens allocated to each of three internal surface preparation groups: 1) burr; 2) OSCAR; or 3) no treatment. Internal holes were re-cemented, then each specimen was cut into 5mm discs. Shear testing of discs was completed by a technician blinded to original grouping (Instron 5567, UK), recording ultimate shear strengths.

The mean shear strength for OSCAR-prepared specimens (17 MPa, 99% CI 14.9 to 18.6, SD=4.0) was significantly lower than that measured for the control (23 MPa, 99% CI 22.5 to 23.7, SD=1.4) and burr (23 MPa, 99% CI 22.1 to 23.7, SD=1.9) groups (P<0.001, one-way ANOVA with Tukey's post-hoc analysis). There was no significant difference between control and burr groups (P>0.05).

Results show that cement removal technique impacts on final cement-cement bonds. This in vitro study shows a significantly weaker bond when using OSCAR prior to re-cementation into an old cement mantle, compared to cement prepared with a burr or no treatment.

These results have implications for surgical practice and decision-making about specific cement removal techniques used during cement-in-cement revision arthroplasty, suggesting that the risks and benefits of ultrasonic cement removal need careful consideration.

Background

Locking internal fixation through a relatively small surgical dissection presents an innovative technique for managing distal tibial extra-articular fractures.

The aim of this study is to evaluate the biomechanical properties of one locking internal fixation plate used to treat these injuries.

The first metatarsophalangeal (MTP) joint is the key joint of the foot in terms of function during gait. Various replacement toe joint prostheses are commercially available but unlike other replacement joints such as the hip or knee, few simulator based studies have been conducted to evaluate the performance and reliability of these prostheses. Presented are results obtained using a newly developed and validated multi-station MTP joint test-rig that is able to simulate the natural biomechanics of the toe joint. The developed simulator is a multi-station computer controlled electro-pneumatic metataso-phalangeal joint simulator that applies dynamic loading and motions commensurate with the walking gait cycle. This involves the combination of plantar-dorsi flexion range of 32 degrees, 5 degrees of inversion/eversion and toe-off dynamic loading peaking at up to 820 N. Presented are the validation and in vitro test results of MTP joint simulations carried out on silastic and articulating metal and polymer designs of MTP prostheses. Despite being subjected to a reduced loading regime of 300 N peak force, the silastic prostheses were found to perform poorly in the simulator, ultimately failing due to a combination of fatigue crack growth and joint collapse. This behaviour and failure mode was consistent with that of ex vivo origin silastic MTP prostheses examined and provides confidence in the validity of the simulator.

Introduction: There is a recent trend of using a raft of small diameter 3.5 mm cortical screws instead of the large diameter 6.5mm screws in depressed tibial plateau fractures. Our aim was to compare the biomechanical properties of these two constructs in the normal and osteoporotic synthetic bone model.

Methods: 20 rigid polyurethane foam blocks with a density simulating osteoporotic bone and normal bone were obtained. A Schatzker type 3 fracture was created in each block. The fracture fragments were then elevated and supported using 2, 6.5mm cancellous screws or 4, 3.5mm cortical screws.

The fractures were loaded using a Lloyd’s machine and a load displacement curve was plotted.

Results: Osteoporotic model. The mean force needed to produce a depression of 5mm was 700.8N with the 4-screw construct and 512.4N with the 2 screw construct (p=0.007).

Non-osteoporotic model. The mean force requires to produce the same depression was 1878.2N with the 2-screw construct and 1938.2N with the 4 screw construct (p=0.42).

An increased fragmentation of the synthetic bone fragments was noticed with the 2-screw construct but not with the 4-screw construct.

Conclusion: A raft of 4, 3.5 mm cortical screws is biomechanically stronger than two, 6.5mm cancellous screws in resisting axial compression in osteoporotic bone.

To identify mechanisms of failure in plate and nail fixation in proximal humerus fractures. 5% of the proximal humerus fractures need surgical fixation, which is carried out, principally, by open reduction and internal fixation or closed reduction and intramedullary nailing. Fixation failure remains a problem. This study answers the mode of failure of these implants regardless of the fracture personality. In-vitro testing of proximal humerus fixation devices was undertaken in 30 simulated osteoporotic bone models. Fracture-line was created at the surgical neck of humerus in all samples and fixed with five fixation devices; three plating and two nailing devices. The samples were subjected to failure under compression and torque. Failure was achieved in all models. Three failure patterns were observed in torque testing:

The two conventional plates Cloverleaf and T-plate behaved similarly, failing due to screw pull-out from both the proximal and distal fragment with a deformed plate.

The failure mode is dependent on implant properties as well, independent of the fracture personality. It is important to recognise the potential points of failure (proximal or distal fragment) when making the choice of implant to avoid fixation failure.

Introduction and aims: There is a recent trend of using a raft of small diameter 3.5 mm cortical screws instead of the large diameter 6.5mm screws in depressed tibial plateau fractures (Schatzker type 3). Our aim was to compare the biomechanical properties of these two constructs in the normal and osteoporotic sawbone model.

Methods: 10 sawbone (solid rigid polyurethane foam) blocks with a density simulating that of an osteoporotic bone and 10 blocks of a density simulating normal bone were obtained. A Schatzker type 3 fracture was created in each block. The fracture fragments were then elevated and supported using 2, 6.5mm cancellous screws in 10 blocks and 4, 3.5mm cortical screws in the remaining.

The models were loaded to failure using a Lloyd’s machine. A displacement (depression) of 5mm was taken to be the point of failure. A load displacement curve was plotted using Nexygen software and the force needed to cause a depression of 5mm was calculated in each block. Mann Whitney U test was used for statistical analysis.

Results: Osteoporotic model

The mean force needed to produce a depression of 5mm was 700.8N with the 4-screw construct and 512.4N with the 2 screw construct. This difference was statistically significant (p=0.007).

Non-osteoporotic model

The mean force requires to produce the same depression was 1878.2N with the 2-screw construct and 1938.2N with the 4 screw construct. The difference was not statistically significant (p=0.42).

An increased fragmentation of the sawbone fragments was noticed with the 2-screw construct but not with the 4-screw construct.

Conclusion: A raft of 4, 3.5 mm cortical screws is biomechanically stronger than 2, 6.5mm cancellous screws in resisting axial compression in osteoporotic bone.

Background: The usefulness of the Nottingham Health Profile as a generic quality of health outcome measure has been described in a number of Orthopaedic conditions. This study was done to compare two quality of life questionnaires, the Nottingham Health Profile (NHP) and the Oswestry Disability Index (ODI) regarding the internal consistency, validity and responsiveness as outcome measures in patients undergoing surgery for lumbar nerve root decompression. We also assessed the effects of smoking, type of lesion, clinical presentation, number of levels involved, operating surgeon and duration of symptoms.

Methods: 37 patients with clinical nerve root entrapment, confirmed radiologically were treated with decompression surgery by two surgeons. We used the NHP and the ODI to assess the severity of symptoms prior to and at 3 and 8 months following surgery

Results: We were unable to detect factors predictive of better healthcare outcome scores after surgery for sciatica. There was a statistically significant improvement in the total ODI score and all NHP domain scores within the whole cohort, after treatment. NHP had a greater responsiveness in detecting improvement for pain and physical ability as measured by the effect size. Cronbach’s alpha for reliability of scores was consistently above the acceptable threshold of 0.90 for NHP scores and consistently below 0.90 for ODI scores. A “floor and ceiling” analysis revealed that the NHP consistently skewed scores at 3 months post op towards a better outcome compared to the ODI.

Conclusion: The generic Nottingham Health Profile appears to be a more sensitive health questionnaire than the Oswestry Disability Index in assessing the outcome of nerve root decompression surgery. We were unable to identify factors predictive of better outcomes using these scores as outcome measures.

Introduction: There has been a renewed interest in metal-on-metal bearing for total hip replacement with the benefit of a larger head size and decreased incidence of dislocation. In the revision hip scenario cementation of a polyethylene liner, for a previously compromised liner fixation mechanism into a preexisting well-fixed shell or a cage, has become an accepted method to decrease the morbidity of the procedure. Perhaps Bir-mingham cementless cups could be used as cemented devices in primary and revision hip surgery where a cementless cup is not possible.

Aim: To study the pull-out strength of cemented Bir-mingham sockets in an experimental model.

Materials and Methods: Eight Birmingham cups were cemented into wooden blocks after they were reamed to the appropriate size allowing for a 3mm cement mantle, multiple holes drilled into the reamed sockets and cement vacuum-mixed. Cable was then threaded through the holes on the rim of the cup and the wooden block was then mounted on a metal plate and secured. Linear tension was then gradually applied on the cup through the cable.

Results: The pull-out strength of the cemented Birming-ham cups was higher than the failure of the cable. The tensile load to failure for the cables ranged from 3642.6 N to 4960 N with an average load of 4286.9 N.

Conclusion: The average tensile load of 4286.9 is very high compared to previous studies with cemented poly-ethylene and metal liners. This finding is very promising and might support clinical application in complex primary and revision total hip replacement.

Purpose: To biomechanically compare the stability provided by 2, 6.5mm cancellous screws versus that provided by 4, 3.5mm cortical screws in depressed tibial plateau fractures

Methods: We obtained 20 sawbone(solid polyurethane foam) blocks of dimensions 9x6.5x3.5cm. 10 of these blocks had a density of normal cancellous bone (20pcf) and 10 represented osteoporotic bone (10pcf). We created a depressed fracture(Schatzker type 3) using a coring saw. Each fracture had 4 depressed fragments, which were elevated and supported using either 2,6.5mm cancellous screws or 4, 3.5mm cortical screws. The fractures were loaded axially using the Lloyds materials testing machine. A force displacement curve was plotted. A depression of 5mm was considered to be failure.

Results: Osteoporotic model|The mean force needed to produce a depression of 5mm was 700.8N with the 4-screw construct and 512.4N with the 2 screw construct. This difference was significantly significant (p=0.007). |Non-osteoporotic model|The mean force requires to produce the same depression was 1878.2N with the 2-screw construct and 1938.2N with the 4 screw construct. The difference was not statistically significant (p=0.42).

Conclusions: A raft of 4, 3.5 mm cortical screws was biomechanically stronger than 2, 6.5mm cancellous screws in resisting axial compression in the osteoporotic bone model. There was no significant difference in the 2 constructs in non-osteoporotic bone model.

Introduction: The wear of orthopaedic biopolymers is recognised as a major factor in the failure of total joint replacements. Clinical wear data exists for acetabular cups manufactured from three biopolymers: ultra high molecular weight polyethylene (UHMWPE); poly tetra fluoro ethylene (PTFE); and polyacetal. The aim of this paper was to wear test these biopolymers and compare the results with clinical data.

Method and Materials: The biopolymers were tested using a modified, four-station, pin-on-plate wear rig [1]. In the tests, two of the four stations applied reciprocating motion and two applied multi-directional motion. Biopolymer pins articulated against stainless steel plates under a load of 40N. The lubricant consisted of 25% bovine serum and 75% distilled water. A standardised cleaning and weighing protocol was followed, and the biopolymer wear factors were calculated by dividing the volume lost by the product of the load and the sliding distance.

Discussion and Conclusions: Failed and retrieved UHMWPE acetabular cups have been reported as having a clinical wear factor of 2.1 x 10⁻⁶mm³/Nm [2]. However, UHMWPE cups which have been functioning well until removal at post-mortem have been said to show 45 to 69% less wear than revised UHMWPE cups [3]. Combining these values suggests clinical wear factors for functional UHMWPE in the range of 0.95 to 1.45 x 10⁻⁶mm³/Nm. This range fits well with the value of 1.1 x 10⁻⁶mm³/Nm shown in table 1 for UHMWPE under multi-directional motion. A clinical wear factor of 37 x10⁻⁶mm³/Nm has been calculated for PTFE acetabular cups [4]. When compared with the mean wear factor for PTFE pins under multi-directional motion obtained from the pin-on-plate rig, the match is remarkable. For polyacetal cups a mean volumetric wear of 136mm³/ year has been reported [5] and it has been calculated that explanted hip prostheses averaged 1.54 million cycles/year [2]. In polyacetal acetabular cups of 37mm diameter, an average sliding distance of 25mm/cycle can be calculated [6] and it has been said that an equivalent static load of 1000N applies [7]. Taking these four values permits a clinical wear factor for polyacetal cups of 3.5 x 10⁻⁶mm³/Nm to be calculated. This number compares well with the value of 3.8 x 10⁻⁶mm³/Nm seen for the polyacetal test pins under multi-directional motion. In summary, all three biopolymers subject to multi-directional motion exhibited clinically relevant values of wear.

Aim: To compare of strength of constructs using the newer and part specific nail systems: Polarus and European Humeral Nail with that using PHILOS and Conventional plate systems in a simulated 2-part fracture of proximal humerus, in an osteoporotic bone model.

Materials and Methods: A Biomechanical laboratory study was undertaken. Third generation composite Humerus model was used, with short e-glass epoxy fibres forming cortex and polyurethane cancellous core. Low-density polyurethane core (1.2gm/cc) was used to simulate an osteoporotic model. Osteotomy at surgical neck of humerus was carried out to create 2-part fracture of proximal humerus. Samples were fixed using one of the implants- the Polarus nail, the European Humeral Nail, PHILOS Plate, Clover Leaf Plate or T-Plate. Following fixation samples were placed in a custom made jig to fix proximal and distal ends without interfering with implants and osteotomy site. All samples were subjected to cyclical torque, torque to failure, cyclical compression and Compression loading to failure.

Results: The two Nail systems that are specifically designed for fixation of proximal humerus fracture provided significantly better fixation in all the test modalities. PHILOS construct shows less plastic deformation in cyclical torque and cyclical compression when compared to the other plates but the 2 nail systems were far superior. Locking screws did not ‘back off’ in any of the experiments involving the Polarus, European Humeral Nail and PHILOS construct, however ordinary screws used with the conventional plates did back off both in ‘torque and compression’ testing.

Conclusions: Polarus and European Humeral Nail constructs provide better stability in torque and compression as compared to PHILOS, which in turn is a more stable construct in comparison to conventional plating devices.

Introduction: Aseptic loosening is the commonest complication of cemented total hip arthroplasy. Gaseous voids within the cement mantle are thought to act as stress concentrators and points of origin and preferential fracture propagation at the cement stem interface. Assuming a bone tempereature of 37°C, Bishop recommended heating the prosthesis to 44°C, thereby effecting a reduction in cement-prosthesis interface porosity.

The aim of this study was to (I) determine the intra-operative temperature of the femoral cancellous bed prior to insertion of prosthesis, (II) to investigate whether the magnitude of the temperature gradient effects interface porosity (III) to develop clinically relevant recommendations.

Materials and Methods: (I) The intra-operative determination of femoral cancellous boney bed temperature. Sterile, single use thermocouples (Mon-a-therm) were used to record interface temperature in six patients, after canal preparation and lavage. (II) A simulated femoral model was designed consisting of a waterbath, set at temperature determined by (I) with an inner water-tight chamber formed by 19mm diameter polyethylene tubing. Cement (Palacos) was non-vacuum mixed (to exaggerate porosity) for 1 minute and injected in a retrograde manner into the inner tube at 3 minutes. Femoral stems (Exeter) were pre-heated in a second waterbath to 18, 32,35,37,40,44°C, were thoroughly dried and lowered into the inner tube by a Lloyd universal testing machine via a custom jig. The cement was left to polymerise.

The cement mantle was sectioned transversely, then longitudinally to expose the cement-prosthesis interface. This was stained with acrylic dye to facilitate image analysis. Three mantles for each temperature were produced.

Results: (I) The mean femoral canal temperature was 32.3°C, (II) the effect of stem temperature on interface porosity is shown in fig1.

Conclusions: Bone temperature is 32°C after canal preparation using contemporary cementing techniques. Heating to 35°C reduces interface porosity, heating to 40°C is optimal.

Aim: To investigate in vitro the mechanical stability of a locking compression plate (LCP) construct in a simulated diaphyseal fracture of the humerus at increasing distances between the plate and bone.

Materials & Method: A series of biomechanical in vitro experiments were performed using Composite Humerus Sawbone as the bone model. Osteotomy created in the mid-diaphyseal region. A 10mm osteotomy gap was bridged with a 7-hole 4.5 stainless steel plate with one of four methods: a control group consisted of a Dynamic Compression Plate applied flush to the bone and three study groups which comprised of a LCP applied flush to the bone, at 2mm and at 5mm from the bone. Standard AO technique used with locking head screws used for LCP fixation. Static and dynamic loading tests performed in a jig with the bone model fixed both proximally and distally. Samples were subjected to cyclical compression, compression load to failure, cyclical torque and torque to failure. Plastic deformation and failure was assessed. Scanning electron microscopy of the plate and screw surface allowed detailed inspection of micro-fracture in areas of fatigue.

Results: Consistent results were achieved in LCP constructs in which the plate was applied at or less than 2mm from the bone. When applied 5mm from the bone the LCP demonstrated significantly increased plastic deformation during cyclical compression and required lower loads to induce construct failure.

Conclusion: In our laboratory model a significant decrease in axial stiffness and torsional rigidity becomes evident at a distance of 5mm between plate and bone.

The aim of this study was to improve data collection, audit and research data by integrating a comprehensive patient data collection database into the day to day running of an orthopaedic department.

The day to day processes of the house surgeon and registrar junior staff were analysed and tasks identified that would allow accurate recording and recall of orthopaedic-specific patient data by automating and performing tasks that would improve junior staff efficiency. A database was then designed and implemented with a “front end” that performed such tasks as generating operating lists, tracking ward locations of patients, producing discharge summaries and auditing complications. This database was then introduced on the hospital intranet and the “back end” constructed to gather accurate patient and injury data to allow improved data collection and research.

Information and data collected was a significant improvement on previous methods available to the department. Junior doctor compliance for data entry was high but the system needed monitoring and “cleaning” on a weekly basis to maintain its accuracy. This method of data collection was more accurate than anything available in the hospital and has been surprisingly useful in producing data to support concerns within the department regarding managerial changes in hospital systems.

Use of a database collection system that gathers information by performing day to day tasks for junior staff has been an effective and reasonably accurate method of obtaining useful patient data. It requires regular monitoring to be most effective but has been easily integrated and accepted within the hospital system.

New generation alumina-on-alumina (A-A) prostheses have been introduced to try and overcome the problem of osteolysis often attributed to polyethylene wear particles liberated within conventional metal-on-ultra high molecular weight polyethylene (UHMWPE) joints. This study uses a hip simulator to compare the volumetric wear rates of five different radial clearances of A-A joints. Atomic force microscopy (AFM) provided topographic characterisation of the prosthesis surfaces throughout the wear test.

Materials and methods: The wear test was performed on the Durham hip joint wear simulator. The 28 mm diameter alumina ceramic couples investigated were manufactured by Morgan Matroc Ltd., in accordance with ISO 6474. Four radial clearances (33, 40, 48 and 74 μm) of A-A joints were tested to one million cycles with 25% new-born calf serum as the lubricant. Contact mode AFM (TopoMetrix Explorer SPM) was used to produce a topographical map of the poles of the four alumina heads every 0.1 million cycles. Every 0.2 million cycles the wear was assessed gravimetrically using a Mettler AE 200 balance (accurate to 0.1 mg).

Results and discussion: There was no measurable wear of either the heads or cups during this one million cycle wear period. Throughout the wear test the alumina equiaxed grain structure became apparent on the AFM images, the mean alumina grain size was 2 μm. The grain surfaces were below the mean femoral head surface height. Such topography was not observed on an as-received head. Some granular pull-out also took place. As the wear test proceeded, the average area surface roughness increased from 2.33 nm to 4.42 nm for the heads but stayed relatively constant for the cups (from 2.75 nm to 2.97 nm).

Conclusions: The very low wear produced by these A-A hip joints is very difficult to measure gravimetrically as it is close to the limits of resolution of the weighing equipment. The surface topography analysis, however, shows changes to the ceramic surfaces during the wear test and gives an indication of the wear processes and lubrication regimes acting within such joints. The authors wish to thank EPSRC for funding this research and Morgan Matroc Ltd. for supplying the joints.

Aims: Does PHILOS (Proximal Humeral Internal Locking system) construct provides better þxation than Clover leaf plate and T-plate in a simulated 2-part fracture of proximal humerus, in an osteoporotic bone model?Materials and Methods: Biomechanical laboratory study. Third generation composite Humerus model was used, with short e-glass epoxy þbres forming cortex and polyurethane cancellous core. Low density polyure-thane core (1.2gm/cc) was used to simulate an osteoporotic model. Osteotomy at surgical neck of humerus was carried out to create 2-part fracture of proximal humerus. Samples were randomised to receive one of the implants. Following þxation samples were placed in a custom made jig to þx proximal and distal ends without interfering with implants and osteotomy site. All samples were subjected to cyclical torque, Torque to failure, Cyclical compression and Compression loading to failure. Results were entered in a database. Results: PHILOS provided signiþcantly better þxation in ÔTorque to failureñ experiment. PHILOS construct shows less plastic deformation in cyclical torque and cyclical compression. Locking screws did not Ôback offñ in any of the experiments involving PHILOS construct, however ordinary screws did back off both in Ôtoque and compressionñ testing. Conclusions: PHILOS construct provides better stability in Torque and compression as compared to conventional plating devices, in an osteoporotic bone model.

Polymerisation of PMMA results in a volume change resulting from molecular rearrangement. The calculated maximal volume reduction is approximately 7.6%; however, void growth reduces this to 3–6%. The significance of volume reduction is controversial, in particular with reference to void elimination techniques. Whilst the impact of mixing technique on overall volume change is known, little is understood about the dynamic volumetric changes occurring during the crucial time of cement-bone micro-interlock formation. This study aimed to investigate the volumetric behaviour of bone cement during polymerisation.

Polyethylene tubes were modified to simulate the physical and dimensional constraints of the human femoral medullary canal. The tubes were filled with either hand or enhanced vacuum mixed cement and suspended in a water bath. The residual weight of the cement specimen in water was recorded at 60sec intervals for 30 minutes. The dry weight of the cement is known and the immersed weight can be calculated. Archimedes principal allows calculation of the density and thus the volume of the cement mass throughout polymerisation. The specimens were sectioned, stained and analysed to assess sectional porosity.

In no specimen was it possible to demonstrate overall net expansion, however, hand mixed specimens demonstrated a temporary period of expansion during the early exothermic period. Vacuum mixed specimens demonstrated progressive contraction only. Overall volume change correlated closely with sectional porosity.

The overall volume reduction is strongly influenced by porosity. The temporary expansion observed in porous cement specimens must result from temperature driven growth of voids. This expansion occurs during the crucial period of cement-bone micro-interlock formation, and may therefore enhance attempts at pressurisation. Conversely, progressive volume reduction, as seen with low porosity cement, may impede micro-interlock formation. Successful cementation using vacuum mixed cement may therefore be solely dependent on adequate cement pressurisation.

Cementation is an important part of arthroplasty operations. Recent publication of results of Charnley total hip replacement found a rate of early aseptic loosening of 2.3% at 5 years following surgery across a NHS region. There are possibly many reasons for early loosening but precise reasons are still not fully understood, however, it is felt that cementation technique is very important. There seems to be a number of factors involved such as cement mixing techniques and conditions, physical properties of the cement, cementation and surgical techniques, bone-cement interface as well as bone- prosthesis interface. The purpose of this study was to evaluate the effectiveness of the clinical environment in producing bone cement of predictable mechanical and physical properties, and how those properties compare with published data. The investigation compared samples of bone cement, taken from material prepared and used in surgery with cement samples prepared under controlled laboratory conditions and in accordance with ISO materials testing standards.

During 2000–01, 10 total hip replacements were selected for study. All operations involved the use of CMW1 (DePuy) radio-opaque cement, which was mixed using the Vacumix system. In this cohort, all femoral cementations were performed using an 80g cement mix. After careful preparation of the femoral canal, the scrub nurse mixed the cement in accordance with the manufacturer’s instructions. The cement was introduced into the femoral canal, via a nozzle, using the cementation gun and a pressurizer. Following introduction of cement into the canal, the nozzle and cement contained within, was broken off the gun distal to the pressurizer. Once cured, the cement samples were removed from the nozzle, sectioned, and mechanically tested. Due to this novel sample preparation procedure, the tested cement was expected to exhibit mechanical and physical properties characteristic of the cement present in the femoral canal. Samples of CMW1 (Vacumixed) of similar mass and aspect ratio were produced within the laboratory, in accordance with the manufacturers mixing instructions. PMMA bone cement is a brittle, glassy polymer that is susceptible to stress raisers, such as pores, which can reduce the load carrying ability, which in vivo is predominantly compressive in nature. Published mechanical properties of PMMA bone cement vary somewhat, reflecting the dependence of properties on flaw distribution. The density, which provides a measure of porosity, hardness and ultimate compressive strength of the cement samples was measured and compared.

The surgical samples were found to be very consistent in compressive strength (121 ± 6 MPa), density (1.20 ± 0.02 gcm⁻³) and hardness (23.2 ± 1.6 VHN) and closely matched the mechanical properties of the cement mixed in the laboratory.

This study suggests that for the studied cement and mixing regime, the clinical environment is capable of producing a well-controlled cement product that has reproducible and predictable mechanical properties. Further, the novel sample preparation procedure used suggests that the cement within the femoral cavity should demonstrate equally predictable, mechanical and physical properties.