The ideal acetabular component is characterised by reliable, long-term fixation with physiological loading of bone and a low rate of wear. Trabecular metal is a porous construct of tantalum which promotes bony ingrowth, has a modulus of elasticity similar to that of cancellous bone, and should be an excellent material for fixation.

Between 2004 and 2006, 55 patients were randomised to receive either a cemented polyethylene or a monobloc trabecular metal acetabular component with a polyethylene articular surface. We measured the peri-prosthetic bone density around the acetabular components for up to two years using dual-energy x-ray absorptiometry.

We found evidence that the cemented acetabular component loaded the acetabular bone centromedially whereas the trabecular metal monobloc loaded the lateral rim and behaved like a hemispherical rigid metal component with regard to loading of the acetabular bone. We suspect that this was due to the peripheral titanium rim used for the mechanism of insertion.

Introduction: To quantify the magnitude and incidence of haemodynamic changes that occurs during the fixation of extracapsular proximal femoral fractures when using either intra-medullary or extra-medullary fixation device.

Methods: A prospective group of 31 patients with extra-capsular proximal femoral fractures were randomised to either fixation using an extra-medullary compression hip screw or an intra-medullary hip screw. All patients received a general anaesthetic adhering to a standardised anaesthetic protocol including invasive blood pressure monitoring and arterial blood gas sampling. Trans-oesophageal Doppler probe and monitor was used to record pre-operative hypovolaemia and peri-operative changes in cardiac output, stroke volume and corrected flow time (FTc – a reflection of left ventricular end diastolic pressure) during placement of implants.

Results: 77% of patients were hypovolaemic preoperatively, which was corrected with an average of 439 mls of colloid replacement fluid. Application of the extra-medullary CHS produced no change in haemodynamic function. However on insertion of the IMHS we found a statistically significant reduction in stroke volume, cardiac output and FTc without changes in pulse rate or mean arterial pressure. The changes were transient with normal cardiac function returning by 5 mins post operatively.

Conclusion: The transient fall in cardiac function during insertion of the intramedullary hip screw may be caused by fat embolism entering the venous circulation. As these changes are not detected with standard non-invasive monitoring we would recommend that intramedullary devices be used with caution in elderly patients who tend to have poor physiological reserve.

Introduction: Aligning the tibial tray is a critical step in total knee arthroplasty (TKA). Malalignment, (especially in varus) has been associated with failure and revision surgery. While the link between varus malalignment and failure has been attributed to increased medial compartmental loading and generation of shear stress, quantitative biomechanical evidence to directly support this mechanism is incomplete. We therefore constructed a finite element model of knee arthroplasty to test the hypothesis that varus malalignment of the tibial tray would increase the risk of tray subsidence.

Methods: Cadaver Testing: Fresh human knees (N = 4) were CT scanned and implanted with a TKA cruciate-retaining tibial tray (Triathlon CR. Stryker Orthopaedics). The specimens were subjected to ISO-recommended knee wear simulation loading for up to 100,000 cycles. Micromotion sensors were mounted between the tray and underlying bone to measure micromotion. In two of the specimens, the application of vertical load was shifted medially to generate a load distribution ratio of 55:45 (medial: lateral) to represent neutral varus-valgus alignment. In the remaining two specimens, a load distribution ratio of 75:25 was generated to represent varus alignment.

Finite element analysis: qCT scans of the tested knees were segmented using MIMICS (Materialise, Belgium). Material properties of bone were spatially assigned after converting bone density to elastic modulus. A finite element model of the tibia implanted with a tibial tray was constructed (Abaqus 6.8, Simulia, Dassault Systèmes). Boundary conditions were applied to simulate experimental mounting conditions and the tray was subjected to a single load cycle representing that applied during cadaver loading.

Results: The two cadaver specimens tested at 55:45 medial:lateral (M:L) force distribution survived the 100,000 cycle test, while both cadaver specimens tested at 75:25 M:L force distribution failed. The finite element model generated distinct differences in compressive strain distribution patterns in the proximal tibia. A threshold of 2000 microstrain was used for fatigue damage in bone under cyclic loading. Both specimens loaded under 75:25 M:L distribution demonstrated substantially larger cortical bone volumes in the proximal tibial cortex that were greater than this fatigue threshold.

Discussion and Conclusion: We validated a finite element model of tibial loading after TKA. Local compressive strains directly correlated with subsidence and failure in cadaver testing. A significantly greater volume of proximal tibial cortical bone was compressed to a strain greater than the fatigue threshold in the varus alignment group, indicating an increased risk for fatigue damage. This model is extremely valuable in studying the effect of surgical alignment, loading, and activity on damage to proximal bone.

Infection following hip arthroplasty although uncommon can have devastating outcomes. Obesity, defined as a BMI of ≥ 30, is a risk factor for infection in this population. Coagulase negative staphylococcus aureus (CNS) is the commonest causative organism isolated from infected arthroplasties. This study was performed to determine if there has been a change in the causative organisms isolated from infected hip arthroplasties and to see if there is a difference in obese patients.

Data on all deep infection following primary and revision hips was obtained from the surgical site infection register from April 1998 to Nov 2007. Case notes were reviewed retrospectively. There were 49 patients with 51 infected arthroplasties; 25 infected Primary THAs and 26 infected Revision THAs. We found a female preponderance in the infected primary and revision THAs (n=30). 63.2% of all patients had a BMI of ≥ 30, compared to only 34.7% of the non infected population (p< 0.0001). Over the period studied, CNS was the most common organism isolated (56.8%) followed by mixed organisms (37.2%) and staphylococcus aureus (25.4%). Multiple organisms were found exclusively in obese patients. In more than half of cases the causative organisms were resistant to more than two antibiotics.

This study shows that over the last 10 years, CNS continues to be the most frequently isolated organism in infected hip arthroplasties. Multiple organisms with multiple antibiotic resistances are common in obese patients. On this basis we recommend that combination antibiotic therapy should be considered in obese patients.

Introduction: Current Literature is sparse with respect to the optimum surgical approach for fixation of a fractured neck of Femur. A cadaveric study has been performed to determine the pattern of innervation of the Vastus Lateralis muscle. Results indicate that a muscle splitting technique may cause more nerve damage than a muscle reflection technique. The purpose of this study was to determine the clinical and neurophysiological effects of two different surgical approaches to the proximal femur.

Methods: Patients were randomised to receive either a Vastus splitting approach or a Vastus reflecting approach to the fractured femoral neck. The contralateral leg was used as the control for neurophysiological investigation. Needle electromyography was performed on both the operated and unoperated limbs within 2 weeks of surgery.

Results: 25 patients were included in the study randomisation. A total of 17 patients completed neurophysiological investigation, 8 in muscle reflection and 9 in muscle splitting groups. There was a significant reduction in femoral nerve conduction velocity compared to the unoperated control side in the muscle split group. This was also the case for amplitude of response measured in the Vastus muscle. The muscle reflection group showed no significant differences in these parameters compared to the unoperated side.

Conclusion: On the basis of the results of this study we recommend a Vastus Lateralis reflecting approach for proximal Femoral fracture fixation. The functional outcome of a muscle splitting approach remains unclear but could be investigated as part of a larger trial.

Introduction: The ideal acetabular component has low wear, permanent fixation and physiological bone loading. Recently trabecular metal has been promoted as reproducing the modulus of trabecular bone with a cementless fixation. The aim of this trial was to see if a monobloc trabecular backed polyethylene acetabular component loaded the pelvis physiologically as a cemented polyethylene component.

Method: Between 2004 and 2006 54 patients were ran-domised to a cemented polyethylene acetabular component versus a monobloc trabecular backed polyethylene acetabular component. The primary outcome measurement was bone density in peri-prosthetic acetabular regions of interest measured preoperatively and post operatively at 6 weeks and 1 year. Secondary outcomes measured were radiographic and functional outcomes (HHS and Oxford score).

Results: Radiographically 8 patients in the trabecular group had a significant gap in zone II which resolved in 6 by 1 year. The cemented group had 3 patients with a radiolucent line (zone 1) at 1 year. HHS and OXFORD scores improved with no significant difference between the groups. Both groups had significant loss of bone density in the ilium and ischium. The trabecular group produced a significant increase in bone density in the superolateral region. The cemented group produced increased bone density in the superomedial region.

Discussions and Conclusions: There is a significant reduction in BMD for both groups in the upper pelvis and ischium in keeping with finite element modelling predictions. The press-fit group relative to the cemented group resulted in decreased BMD in the superomedial peri-prosthetic region. The trabecular monobloc cup therefore behaves more like a rigid cementless shell despite the properties of trabecular metal.

Introduction: Infection following hip arthroplasty although uncommon can have devastating outcomes. Obesity, defined as a BMI of ≥ 30, is a known risk factor for infection in this population. Coagulase negative Staph Aureus (CNS) is the commonest causative organism isolated from infected arthroplasties. This study was performed to determine if there has been a change in the causative organisms isolated from infected hip arthroplasties and to see if there is a difference in obese patients.

Methods: Data on all deep infection following primary and revision hips was obtained from the surgical site infection register from April 1998 to November 2007. All case notes were reviewed retrospectively.

Results: There were 49 patients with 51 infected arthroplasties; 25 infected Primary THAs and 26 infected Revision THAs. We found a female preponderance in the infected primary and revision THAs (n=30). 65.3% of all patients had a BMI of ≥ 30. Over the period studied, Coagulase negative Staph was the most common organism isolated (56.8%) followed by mixed organisms (37.2%) and Staph Aureus (25.4%). Multiple organisms were found exclusively in obese patients. In more than half of cases the causative organisms were resistant to ≥ 2 antibiotics.

Discussion: This study shows that over the last 10 years, CNS continues to be the most frequently isolated organism in infected hip arthroplasties. Multiple organisms with multiple antibiotic resistances are common in obese patients. On this basis we recommend that combination antibiotic therapy should be considered in obese patients.

Introduction: Tutobone is a solvent-preserved cancellous bovine bone substitute. There is little published about its use in humans. We have been using it as a wedge graft to correct deformity in hindfoot fusion surgery.

Aim: To review the outcome following the use of Tutobone in hindfoot fusion and compare it with a control group without Tutobone.

Method: We performed a retrospective review of all hindfoot fusion performed by the senior author (AG) from 1 Sep 2004 to 31 Jan 2008. We excluded all revision procedures for non-union or malunion. A CT or MRI scan was performed to assess union and graft incorporation in the Tutobone patients at more than six months postoperatively. In the control group fusion was assessed with plain radiographs. The difference in proportion of fusion with complete fusion by six months post-operatively was assessed with a Fisher’s exact test

Results: There were eleven patients in the Tutobone group (1 ankle, 7 subtalar and 3 triple fusions) and 35 in the control group (15 ankle, 11 subtalar, 3 pantalar and 6 triple fusions). All Tutobone patients had partial union on CT/MRI scans. The Tutobone graft had not incorporated at a mean time interval of 14 months post surgery. 30 out of 35 control patients had fused by six months and 33 out of 35 controls were fused by 12 months. The rate of complete fusion between the two groups at six months was statistically significant (p< 0.0001). Two Tutobone patients developed an inflammatory reaction at more than six months post fusion. This reaction is not infective and appears to be a reaction to the Tutobone.

Conclusion: Tutobone should not be used in hindfoot fusion surgery.

Idiopathic anterior knee pain (AKP) is common in adolescents and young adults. Most believe that the origin of the problem lies in the patello-femoral joint. Hamstring tightness has also been attributed as an important cause.

The aim of our study was to compare biometric parameters in patients with idiopathic AKP and controls. We also wanted to assess whether there was a difference in the relative electromyographic (EMG) onset times of the medial and lateral hamstrings.

We prospectively recruited patients with idiopathic anterior knee pain in the age group 11 to 25. Patients, but not the control population, had AP, lateral and skyline radiographs taken to rule out other pathology.

We had 34 patients (60 knees) with a minimum one year follow up. There was no difference in the symptoms of patients who attended physiotherapy as compared to those who did not. Patients with knee pain had significantly more hip external rotation (63 deg) as compared to the control (47 deg) group (p=0.001). Patients also had significantly more hamstring tightness (p=0.04).

Surface EMG was recorded (17 patients and controls each) from the medial and lateral hamstrings during 3 repetitions of a maximal voluntary isometric contraction exercise with the knee at 45° of flexion. The lateral hamstrings contracted 48.7 m.sec earlier than the medial hamstrings in patients as compared to controls.

AKP is a multifactorial and self-limiting disorder. Earlier contraction of the lateral hamstrings may cause tibial external rotation and contribute to the symptoms. Our data suggests that physiotherapy did not significantly alter the course of the condition. We believe that increased hip external rotation may contribute to the symptoms by increasing medial facet stress.

Introduction: Several institutes in continental Europe and the US now prescribe low molecular weight heparin for patients with ankle fractures being treated in a below knee plaster cast. Jorgensen et al reported an incidence of deep venous thrombosis (DVT) of up to 20% in patients treated in a cast. However, their study included patients with variable diagnoses, ranging from tendon ruptures to fractures. The aim of our study was to assess the incidence of DVT in patients with ankle fractures that have been treated conservatively in a below knee cast.

Method: We performed an ultrasound scan on patients with conservatively ankle fractures at the time of removal of the cast. The same ultrasound technician performed all the scans. The local regional ethics committee had approved this study.

Results: So far we have performed an ultrasound scan on 98 patients with ankle fractures. We are likely to complete the study in November 2006 (120 patients). We have encountered only 2 below knee DVTs (2.04 %). None of them involved the popliteal vein. Both patients were completely asymptomatic and were full weight bearing in the cast. A repeat scan showed no evidence of progression. None of the patients had an above knee DVT or a pulmonary embolism.

Discussion: The risk of deep venous thrombosis is said to be higher in patients with a plaster cast because of the decreased ability of the calf muscles to pump the venous blood back to the heart. Though some studies have indicated an incidence of up to 20%, the incidence in our population was only about 2%Thus, DVT is a rare event in patients with ankle fractures. Though it is a serious event, its rarity does not justify a blanket prophylaxis regimen for all patients with ankle fractures.

Introduction: Removal of well-fixed, cementless, acetabular components after resurfacing hip arthroplasty remains a challenging problem. Damage to host bone may limit options for reconstruction, compromise the long-term result of the revision operation and fundamentally defeat the aim of bone conserving resurfacing hip surgery.

Methods: A series of 6 consecutive patients who under-went removal of a secure, acetabular resurfacing component at the time of revision arthroplasty were included for review. During the operative procedure, the size of the component which was removed and the diameter of the final reamer used prior to implantation and final acetabular implant were recorded. The modification of the standard explant technique will be described which allows safe removal of any size of acetabular component.

Results: In all patients the indication for index arthroplasty was osteoarthritis. Three cases were MMT (Smith and Nephew), 2 Cormet 2000 (Corin, UK). and 1 DUROM (Zimmer). The indications for acetabular revision were infection in all cases. The median difference between the size of component removed and the size of final component implanted was 4 mm.

Discussion: Our modification uses a pre-existing system. The ease of removal with this modification and the lack of any further damage to the host bone illustrates that the Explant Acetabular Cup Removal System can be safely expanded to removal of well fixed resurfacing monoblock acetabular components. With experience, any manufacturers resurfacing shell can be removed with virtually no bone loss.

Introduction: Poor proprioception and imbalance between quadriceps and hamstrings have been suggested as causes for anterior knee pain. The aim of our study was to compare the proprioception of patients with anterior knee pain to a normal population and to compare the activity of quadriceps and hamstrings using electromyography (EMG) in the 2 groups.

Methods: Patients and controls between the ages 11–25 yrs were recruited into the study. The proprioception (stability index) of the patients and controls was tested using the Biodex stability system. This computerised system tests the ability of a person to balance his/her own body on a platform that moves in various directions. Surface EMG was recorded from the quadriceps and hamstrings during this test. EMG was also recorded as the patients and controls stepped onto a 20cm step. EMG activity was normalised to levels elicited during maximal isometric contractions.

Results: 18 patients and 27 controls were recruited.

We found no significant difference between the groups in the EMG intensity of vastus lateralis relative to biceps femoris, or vastus medialis relative to vastus lateralis, during the balance test or during the step up task (Mann Whitney U test all p> 0.05). We did not find any difference in the proprioceptive abilities of the two groups

Conclusion: We found no significant difference between the groups in the intensity of muscle contraction of the hamstrings relative to the quadriceps, i.e. no evidence for an imbalance in the patients. However the temporal relation between the two needs further investigation.

The aim of this study was to determine whether there is a difference in the functional outcome between fixed and mobile bearings in total knee arthroplasty.

120 patients were randomized (computer generated) to receive either a fixed or mobile bearing P.F.C. Sigma total knee replacement. 96 patients were needed to detect a 20° difference in range of motion (ROM) with a significance level of 0.05 and a test power of 0.97. Oxford knee score (OKS) and ROM were assessed independently before and one year after surgery.

Mean ROM and Oxford knee score before and at one year after surgery for both groups are shown as preliminary results for 70 patients (follow-up expected to be completed by March 2006):

There is no statistically significant difference in the mean ROM at one year and in change in ROM between the two groups (p=0.53 and p=0.21 respectively). The findings were similar for Oxford Knee Score at one year and change in Oxford Knee Score (p=0.45 and p=0.82). There was no early aseptic loosening in either group.

The one year results suggest that there is no significant difference in functional outcome measured as ROM and Oxford Knee Score between the two types of bearing. Further follow-up will be carried out to detect any differences in the long term outcome.

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.

Aim: We have carried out a prospective study to compare duplex ultrasonography and transcutaneous oxygen tension as predictors of wound healing after knee arthroplasty.

Methods and Materials: 53 patients were included in the study. All underwent pre-operative duplex scans of their lower limbs. In addition, transcutaneous oxygen tension measurements were made adjacent to the proposed incision pre-operatively and on days 1, 3 and 7 post knee arthroplasty. Wound healing was assessed using the ASEPSIS wound score.

Results: 4 wounds had evidence of delayed wound healing. Duplex ultrasonography was a poor predictor of such problems; however there was a significant correlation between pre-operative transcutaneous oxygen tension and post-operative wound scores.

Conclusion: Pre-operative transcutaneous oxygen tension measurement is a better predictor of wound healing after knee replacement surgery than duplex ultrasonograph, and may be a useful adjunct to the investigation of patients at risk of wound problems post knee arthroplasty.

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.

Femoral head deformity with flattening and lateral protrusion can occur secondary to epiphyseal dysplasia or avascular necrosis of any aetiology in childhood. This causes painful impingement of the lateral femoral head on the acetabular lip, a phenomenon known as hinge abduction. We aimed to review our experience of valgus extension osteotomy in the treatment of hinge abduction in children and young adults with avascular necrosis.

Twenty patients undergoing valgus osteotomy for hinge abduction performed by a single specialist were clinically and radiologically reviewed. The aetiology was Perthes disease in 16 patients and treatment of DDH in 4 patients. The indication for the procedure was pain and limited abduction. The mean follow-up was 4.5 years. Patients were assessed using modified Iowa hip scores at final follow-up. The procedure corrected some leg shortening and improved the abduction range of the affected hip. Overall 80 % of patients did well. The mean Iowa hip score in Perthes group was 84 at final follow-up.

Four patients preoperatively had cysts/ defects in their femoral head. These were seen to fill up during their postoperative follow-up. Poorer outcome was associated with preoperative hip stiffness and surgery before stabilisation of the avascular process.

Conclusion: Valgus osteotomy is an effective procedure for relieving hinge abduction with associated additional benefits including improvement of leg shortening and improvement in hip abduction. The procedure should be avoided in stiff hips.

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.

Purpose: To compare the analgesia provided by pure subcutaneous infiltration (Gale technique) of lignocaine with that provided by infiltration of lignocaine into the carpal tunnel in addition to the subcutaneous tissue (Altissimi technique) for carpal tunnel decompression

Methods: 20 patients with bilateral carpal tunnel syndromes were chosen for the study. Patients were randomised to receive one local anaesthetic technique on one side and the other on the other side. The pain scores were recorded intraoperatively and 2 and 4 hours postoperatively.

Results: 5 patients experienced intra-operative pain with the Gale technique, while one did with the Altissimi technique (p=0.15 using Mann Whitney U test). Postoperative analgesia at 2 hours was significantly better with the Altissimi technique (p= 0.009). Patients with the Altissimi technique also required less number of analgesic tablets over 24 hours post surgery (p=0.01).

Conclusions: We found no statistically significant difference in the intra-operative pain scores with the two techniques. However, postoperative pain relief was much better with the Altissimi technique.

Complications after total knee arthroplasty (TKR) such as malalignment, instability, subluxation, excessive wear, and loosening have been attributed to poor soft-tissue balance. Traditional approaches for soft-tissue balance involve static measurements in full extension and at 90° flexion. A trial prosthesis instrumented with force transducers was used to measure soft-tissue balance through the entire range of flexion.

The trial prosthesis was instrumented with four force transducers, one at each corner of the tibial tray, and was implanted in four cadaver knees and four patients intra-operatively. Tibial forces were recorded during passive knee flexion after the tibial and femoral bone cuts were made and again after soft-tissue balance was achieved using standard techniques.

In all eight knees measurable imbalance was initially recorded. The differences in forces were a mean of 18 N (range, 6 to 72) mediolateral and a mean of 26 N (range, 13 to 108) anteroposterior. After a routine procedure of soft-tissue balancing, the mean imbalance between the transducers was reduced by 62 % to 87 % (p < 0.05). However, even the knees that appeared perfectly balanced at 0° and 90° flexion, some imbalance occurred [mean 22 N (range, 2 to 34)] at flexion angles other than 0° and 90°.

Soft-tissue balance in TKR remains a complex concept. Even after accurate static balancing was achieved in extension and 90° flexion, dynamic measurements revealed discrepancies in mid flexion, which may explain the wide variation in knee kinematics reported after TKR and in the reported incidences of mid-flexion knee instability. Computer-aided surgical navigation systems can increase the precision and accuracy of component alignment. However, these systems cannot directly address soft-tissue balance and knee tightness. An instrumented tibial prosthesis could be a useful adjunct to enhance the value of these navigation tools.