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Orthopaedic Proceedings
Vol. 94-B, Issue SUPP_XL | Pages 212 - 212
1 Sep 2012
Walker P Kahn H Zuckerman J Slover J Jaffe F Karia R Kim J
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INTRODUCTION

Total Knee Arthroplasty (TKA) is a durable procedure which allows most patients to achieve a satisfactory functional level, but there can be instability under stressful conditions. Instability is one cause of early revision, often due to misalignment or inadequate ligament balancing. Persistent instability may cause elevated polyethylene wear. Lower levels of instability may cause patient discomfort with certain stressful activities. Hence quantifying instability may have an important role in the functional evaluation of TKA. Several previous studies showed that accelerometers have advantages in kinematic studies including low cost, ease of application, and application to any activity. The aim of this study was to demonstrate the use of an accelerometer attached to the anterior of the tibia, as an evaluation of knee stability of TKA patients. It was postulated that accelerations between TKAs and normal controls will be different, which could indicate abnormal TKA kinematics involving instability, especially for high intensity activities.

METHODS & MATERIALS

We tested 38 TKA knees in 27 patients, in the age range of 50–80 years, with a minimum follow up of 6 months; and 25 knees in 16 shoulder patients, who had no known knee pathology as age-matched controls. A tri-axial accelerometer was firmly attached to the anterior proximal tibia to measure 3-axis accelerations with a sample rate of 100 Hz. Four activities were tested;

Starting with the test leg, walk 3 steps then come to a sudden stop

Take one step forward with the non-tested leg and make a tight 90∗∗∗∗∗ turn towards the non- tested knee direction

Sit down for 3–4 seconds then stand back up

Step up on a 7″ inches high box with the test leg, followed by the non-test leg. Then step down from the box with the test leg, followed by the non-test leg.

During the activities, the patients responded to a questionnaire on instability and pain for each activity. For each test at the time of foot impact, there was a high/low peak acceleration, the peak-to-valley being taken as the indicator. The mean total magnitude of the acceleration was compared between the TKA and control groups in the anterior-posterior direction using the Student's t-test. Statistical significance was at p-value < 0.05.


Orthopaedic Proceedings
Vol. 94-B, Issue SUPP_XXXVII | Pages 225 - 225
1 Sep 2012
Da Assuncao R Samra D Walker P Bruce W
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Introduction

Pre-operative patient education prior to hip and knee arthroplasty is thought to be beneficial for patients in general, although the clinical effect is unproven. Pre-operative education is now standard practice in many orthopaedic units, including our own. Anecdotally, we found patients in the private sector to be more satisfied with their education than those in the public sector, despite very similar education programs. We set out to investigate this observation and establish whether there were differences in the perception of educational quality between these groups.

Methods

After appropriate power calculation, 60 patients in each group (120 total) were interviewed after hip or knee arthroplasty. Satisfaction with education was assessed on a simple 5 point scale and demographic data including internet access was obtained, as well as Short-Form 12 (SF-12) data to establish physical and mental function. The education program in each group was similar, with written material provided and verbal education given at a single pre-operative session. This included specialist nursing and physiotherapy input. Results were analysed with the chi2 test, Mann-Whitney U test and logistic regression as appropriate, with significance at P<0.05.


Orthopaedic Proceedings
Vol. 94-B, Issue SUPP_XXXVII | Pages 226 - 226
1 Sep 2012
Da Assuncao R Haddad R Bruce W Walker P Walsh W
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Introduction

In order to prepare hamstring autograft, suture fixation to the tendon is required to secure and handle the tendon during harvest and preparation. We use a simple, grasping suture which doesn't require suture of the tendon, thus saving time and avoiding violation of the graft itself. We present this technique, with results of mechanical testing compared to a standard whip suture, traditionally used to handle hamstring autograft.

Methods and materials

Twelve uniform ovine flexor tendons were prepared. A number two braided polyester suture was used in all cases. Six tendons were prepared with a standard, non-locking whip-suture, maintaining uniformity of suture bite and working length between samples. Six tendons were prepared with the utility suture, also taking care to maintain uniformity. The suture was applied by tying the thread around the tendon with a single-throw granny knot then symmetrically wrapping the suture ends from proximal to distal and securing with another single throw, allowing compression of the tendon with longitudinal tension on the suture. All the samples were tested to failure in uniaxial tension in a materials testing machine. Peak load values and load/displacement curves were acquired and results analysed with a two-sample T-test assuming significance at P<0.05.


Orthopaedic Proceedings
Vol. 94-B, Issue SUPP_XXV | Pages 259 - 259
1 Jun 2012
Yildirim G Walker P Conditt M Horowitz S Madrid I
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Introduction

The MAKO Surgical Rio Robotic Arm utilizes the pre-op CT images to plan positioning of the uni-condylar and patella-femoral components in order to achieve the most desirable kinematics for the knee joint. We hypothesize that the anatomic matching surfaces and the cruciate retaining design of the Restoris knee will best replicate normal knee kinematics. We tested the healthy cadaveric knee versus the MAKO knee and the most common TKR designs in order to evaluate and compare the kinematic properties.

Methods

Six healthy male left knees were dissected to leave only the knee capsule and the quadriceps tendon intact. The femur and the tibia were cut 20cm from the joint line and potted with cement into a metal housing. The knee was attached to a crouching machine capable of moving the knee joint though its normal human kinematics from extension to maximum flexion, validated in previous studies. Forces applied to the quadriceps tendon allowed the knee to flex and extend physiologically, and springs attached to the posterior were substituted as the hamstrings at a rate of half the force exerted by the quadriceps as shown in the literature. Three dimensional visual targets attached to the bones were tracked by computer software capable of recreating the positions of the bones in any given flexion angle. A cruciate retaining and posterior stabilized TKR design were chosen to represent the TKRs most commonly available in the market today. The intact knee, MAKO implanted knee, CR and PS TKR designs were tested in sequence on the same specimens. The computer software analyzed the normal distance between the bone surfaces and plotted the locations of contact which could then be quantitatively compared for each given scenario [Fig. 1].


Orthopaedic Proceedings
Vol. 94-B, Issue SUPP_XXV | Pages 95 - 95
1 Jun 2012
Hua J Muirhead-Allwood S Sandiford A Skinner J Walker P
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Total hip replacements have shown great benefits to patients through relief of pain and restoration of function. However, because of the extensive variation in the size and shape of the femoral canal, especially for the situation encountered in the revision hip arthroplasty, standard uncemented hip systems with a limited number of sizes are unable to provide an accurate fit in every case. This study showed clinical results of 112 primary total hip replacements and 158 revision total hip replacements, using custom made CAD-CAM (Computer Aided Design-Computer Aided Manufactured) hip prostheses inserted between 1992 and 1998.

For primary hip replacements, the implants were designed to produce proximal line-to-line fit with the femoral bone and to provide optimal biomechanical environment of the hip. The stem was HA coated, 53 males and 58 females were included. Mean age was 46.2 years (range 24.6yrs - 62.2 yrs). The average duration of the follow up was 24 years (10 – 17 years). The mean Harris Hip Score (HHS) was improved from 42.4 to 90.3, mean Oxford Hip Score (OHS) was improved from 43.1 to 18.2 and the mean WOMAC hip score was improved from 57.0 to 11.9. There was 1 revision due to failure of the acetabular components but there were no failures of the femoral components. In the whole follow-up period, the survival of the femoral stem alone was 100%.

For revision hip replacements, the implants were designed using our design strategies of graduate approach to different revision situation based on Paprosky's classification of femoral bone defect. The implants were HA coated; some of them had distal cutting flutes. A total of 158 patients (97 males and 61 females) who had operation between 1991 and 1998 were followed up, among them 138 cases were due to aseptic loosening, 6 cases were periprosthetic fractures and 14 cases were infection. The average age was 63.1 years (range 34.6 – 85.9 years). The minimum follow up was 10 years (range 10 – 12 years). The mean Harris Hip Score was improved from 44.2 to 89.3, mean Oxford Hip Score was improved from 41.1 to 18.2 and the mean WOMAC hip score was improved from 52.4 to 12.3 respectively. 6 cases required further revision surgery, among them 3 were due to aseptic implant loosening, the overall survivorship at ten years was 97%.

The CAD-CAM hip stems are able to provide optimal implant fixation and restore hip function for every patient regardless their original femoral shape, bone condition and biomechanics of the hip. The excellent medium to long term clinical results justifies the use of CAD-CAM custom hip stems.


Orthopaedic Proceedings
Vol. 94-B, Issue SUPP_IV | Pages 112 - 112
1 Mar 2012
Biant L Bruce W Walker P Herrmann S Walsh W
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‘High flexion’ polyethylene tibial tray inserts are available from total knee replacement (TKR) manufacturers. There is currently no published data available that examines how much extra knee flexion these new implants give or if there are any wear consequences for the change in design. The high flexion inserts are narrower posteriorly than standard inserts and have chamfers anteriorly and on the post in cruciate sacrificing designs.

This prospective randomised controlled trial of 100 patients undergoing posterior stabilised TKR compared knee flexion, measured intra-operatively by a computer navigation system, of the standard and high flexion trial inserts in the same knee. Patients were then randomised to receive either a standard or ‘high flexion’ definitive component and the stability assessed. The post-operative knee flexion of all patients was measured at six months.

High flexion inserts did not give significantly more knee flexion than standard inserts either per-operatively at the trial insert stage, or at six months post-op and resulted in marginally more anterior draw. The average per-operative difference in flexion between standard and high flex inserts measured in the same knee was 3.2° (range -4-18°) The average knee flexion at 6 months post op was 106° for both groups. The average change in knee flexion comparing pre and post op was 2.3° for the high flex group and 0.6° for the standard insert group.

Laboratory Tek scan contact pressure analysis at the surface of the standard and high flexion designs was not significantly different, but the thinner polyethylene of the high flexion design raises questions about wear characteristics. High flexion polyethylene inserts are probably not justified in terms of improved knee flexion, but may be a useful option in certain technical circumstances during TKR such as patella baja or if the patella impinges on the post in deep flexion.


Orthopaedic Proceedings
Vol. 93-B, Issue SUPP_IV | Pages 473 - 473
1 Nov 2011
Iguchi H Watanabe N Murakami S Hasegawa S Tawada K Yoshida M Kobayashi M Nagaya Y Goto H Nozaki M Otsuka T Yoshida Y Shibata Y Taneda Y Hirade T Fetto J Walker P
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Introduction: For longer lasting and bone conserving cementless stem fixation, stable and physiological proximal load transfer from the stem to the canal should be one of the most essential factors. According to this understanding, we have been developing a custom stem system with lateral flare and an off-the-shelf (OTS) lateral flare stem system was added to the series. On the other hand, dysplastic hips are often understood that they have larger neck shaft angle as well as larger anteversion. In other words they are in the status called “coxa valga.” From this point of view we had been mainly using custom stems for the dysplastic cases before. After off-the-shelf lateral flare stem system; which is designed to have very high proximal fit and fill to normal femora; was added, we have been using 3D preoperative planning system to determine custom or OTS. Then in most of the cases, OTS stem were suitably selected. Our pilot study of virtual insertion of OTS lateral flare stem into 38 dysplastic femora has shown very tight fit in all 38 cases. The reason was analyzed that the excessive anteversion is twist of proximal part over the distal part and the proximal part has almost normal geometry. In the present study, 59 femora were examined by the 3D preoperative planning system how the excessive anteversion effect to the coxa valga status.

Materials and Methods: Fifty-nine femoral geometry data were examined by the 3D preoperative planning system. Thirty-three hip arithritis, 3 RA, 2 metastatic bone tumours, 5 AVN, 1 knee arthritis, 12 injuries, and 3 normal candidates were included. Among them one arthritic Caucasian and one AVN South American were included. The direction of the femoral landmarks; centre of femoral head (CFH), lesser trochanter (LTR), and asperas in 3 levels (just below LTR, upper 1/3, mid femur; A1-3); were assessed as the angle from knee posterior condylar (PC) line. Neck shaft angle of each case was assessed from the view perpendicular to PC line and neck shaft angle form the view perpendicular to CFH and femoral shaft (i.e. actual neck shaft angle).

Results: Average anteversion was 34.4 +/−9.9 degree. CFH and LTR correlated well (i.e. they rotate together). A1, A2, A3 correlated well (i.e. they rotate together). LTR and A1 correlate just a little, LTR and A2 were independent each other. So the twist existed around A1. Neck shaft angle was 138.7+/−6.6 in PC line view and in actual view 130.3+/−4.4. No excessive neck shaft angle was observed in actual view. Even the case that has the largest actual neck shaft angle (140.4), the virtual insertion showed good fit and fill with the lateral flare stem.

Conclusion: In many high anteversion cases, coxa valga is a product of the observation from non perpendicular direction to CFH-shaft plane. Selection or designation of the stem for high anteversion cases should be carefully determined by 3D observation.


Orthopaedic Proceedings
Vol. 93-B, Issue SUPP_IV | Pages 427 - 427
1 Nov 2011
Takemoto R Arno S Kinariwala N Chan K Hennessy D Nguyen N Walker P Fetto J
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Over the last two decades, design modifications in cementless total hip arthoplasty have led to longer lasting implants and an increased success rate. However, there remains limitations to the cementless femoral stem implant. Traditional cementless femoral components require large amounts of bone to be broached prior to stem insertion (1). This leads to a decrease in host bone stock, which can become problematic in a young patient who may eventually require a revision operation during his or her lifetime. Osteopenia, only second to distal stress shielding can lead to aseptic loosening of the implant and stem subsidence, which also accelerates the need for a revision operation (24). Recent literature suggests that thigh pain due to distal canal fixation, micro-motion, uneven stress patterns or cortex impingement by the femoral stem is directly correlated to increased stem sizes and often very disabling to a patient (58). In this study, we sought to determine whether reducing stem length in the femoral implant would produce more physiologic loading characteristics in the proximal femur and thus eliminate any remaining stress shielding that is present in the current design. We analyzed the surface strains in 13 femurs implanted with

no implants,

stemless,

ultra short and

short stem proximal fill implants in a test rig designed to assimilate muscle forces across the hip joints, including the ilio-tibial band and the hip abductors.

Analysis of the resulting surface strains was performed using the photoelastic method. For each femur, intact and with the different stem length components in place, the fringe patterns were compared at the same applied loads. The highest fringe orders observed for all tests were located on the lateral proximal femur and medial proximal femur. The fringes decreased as they approached the neutral axis of bending (posterior and anterior). Distal fringe patterns were more prominent as the stem length increased. The results demonstrate that the stemless design most closely replicated normal strain patterns seen in a native femur during simulated gait. The presence of a stemless, ultra short and short stem reduced proximal strain and increased distal strain linearly, thereby increasing the potential for stress shielding. The stemless design most closely replicated normal strain patterns observed in a native femur and for this reason has the potential to address the shortcomings of the traditional cementless femoral implant.


Orthopaedic Proceedings
Vol. 93-B, Issue SUPP_IV | Pages 424 - 424
1 Nov 2011
Fetto JF Walker P
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Recent trends in surgical techniques for THR, i.e. MIS and anterior approaches, have spawned an interest in and possible need for shorter femoral prostheses. Although, previously published clinical investigations with custom short stems have reported very encouraging results (Walker, et al, 1,2), the transition to off-the-shelf (OTS) versions of shorter length prostheses has not met with the same degree of success. Early reports with OTS devices have documented unacceptably high and significant incidences of implant instability, migration, mechanical/aseptic failure, and technical difficulty in achieving reproducible implantation outcomes. They have highlighted the absolute need for a better understanding of the consequences of changes in implant design as well as for improvements in instrumentation and surgeon training.

Several basic questions must be addressed. First, what is the purpose of a stem? Second, can stem length be reduced and if so by how much can this be safely done. Third, what are the effects of stem shortening and are there other design criteria which must take on greater importance in the absence of a stem to protect against implant failure.

To examine these questions a testing rig was constructed which attempts to simulate the in vivo loading situation of a hip, fig.1(Walker, et, al.). Fresh cadaveric femora were tested with the femora intact and then with femoral components of varying stem length implanted to examine the distribution of stresses within the femur under increasing loads as a function of stem length. This was correlated with observations of prospective DEXA measurement of proximal femoral bone mass and implant migration following THR(Leali, 3).

Our studies indicated that a stem is not an absolute requirement in order to achieve a well functioning, stable implant. However in order to reduce the possibility of mechanical failure a reduced stem or stemless implant absolutely must have, inherent to its design, a provision for sufficient contact with both the medial and lateral proximal metaphyseal femur. As well it must also have a flat posterior surface parallel to, and in contact with, the posterior surface of the proximal femoral metaphysis. These conditions will provide support against distal migration as well as bending moments in the A/P plane. As a consequence of this latter condition, appropriate anteversion must be achieved in the neck region of the prosthesis and not by rotation of the implant within the proximal metaphyseal cavity of the femur.

In conclusion, this study demonstrates that simply reducing the length of an existing implant to accommodate changes in surgical techniques may not be a reasonable or safe design change. Such shortened versions of existing stem designs should undergo rigorously in vitro testing before being released for implantation.


Orthopaedic Proceedings
Vol. 92-B, Issue SUPP_II | Pages 294 - 295
1 May 2010
Biant L Bruce W Assini J Walker P Walsh W
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Intro: Anatomical abnormality associated with severe developmental dysplasia of the hip presents technical difficulties at THR. Patients often present at a younger age and may have had previous surgery.

We report the difficulties encountered during surgery, and the long term results of patients who had Crowe 3 or 4 DDH and a technically difficult primary hip arthroplasty using the modular S-ROM stem.

Method: 28 patients were entered into the prospective trial. The average age of the patient at surgery was 45 (range 23–74 years). All patients underwent surgery by the senior author using the S-ROM femoral stem. They were followed up for an average of 10 years (range 5–16 years), clinical scores recorded by a clinician other than the surgeon and radiographs were examined by an independent radiologist.

Results: 21 patients required a significant autologous bone graft, one patient had a large allograft and six patients required femoral shortening at the time of their THR.

4 patients had a technical complication during surgery. The average pre-op Harris Hip Score was 37, at 5 years it was 83, and at 10 years 81. The SF12 measure of physical and mental wellbeing was 43.90 physical/54.48 mental at 5 years, and 41.64 physical/54.03 mental at 10 years. The WOMAC average score (the lower the score the better the outcome) was 27 at 5 years and 23 at 10 years.

None of the S-ROM stems had been revised, 2 hips had undergone acetabular revision and one hip had a liner exchange. None of the S-ROM stems were loose at latest follow-up. Four hips had osteolysis in Gruen zone 1, one hip had osteolysis in zone 7, and one hip had osteolysis in zone 1 and 7. There was no evidence of osteolysis around or distal to the sleeve.

Conclusion: The S-ROM stem used in primary THR shows excellent results at 10 years in patients with anatomical abnormality related to severe DDH.

S-ROM stem/sleeve modularity allows femoral component anteversion independent of the position of best fit in the proximal femur, and helps overcome the technical difficulty in these patients.


Orthopaedic Proceedings
Vol. 92-B, Issue SUPP_I | Pages 98 - 98
1 Mar 2010
Iguchi H Tanaka N Kobayashi M Nagaya Y Goto H Nozaki M Murakami S Hasegawa S Tawada K Yoshida Y Otsuka T Fetto J Walker P
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Since 1993, we have been developing preoperative planning system based on CAT scan data. In early period it was used to decide cup diameter and orientation for Total Hip Arthroplasty (THA). It was done using hemisphere object locating proper position and orientation. According to our progress, we have started using it for custom stem designing, stem selection and stem size planning too since 1995. Since 2001, we have been using it for almost all THA cases. We also have started use it for any case we have question about 3D geometries. Since 2005 we started computer planed 2 staged THA after leg elongation for high riding hips and reported at ISTA 2007 too. Now our policy became that every tiny question we have, we shall analyze and plan preoperatively.

In our population, the incidence of the developmental dysplastic hips is higher. The necks often have bigger anteversion, and less acetabular coverage. So we often use screws for cup fixation. The screw direction allowed in thin shell thickness is limited and less bone coverage makes good cup fixation difficult. With highly defected cases and with revision cases the situation is more difficult.

In the present study, we have developed acetabular 3D preoperative planning method with screw direction, length, and for the cases with defect, cup supporter pre-shaping with models and prediction of the allograft volume.

For the less defect cases, geometries of cup with screw holes were requested to the maker and were provided for us. Screws were attached perpendicular to each screw hole. Screw geometries have marks at every 5mm to plan proper length. The cup was located as much as closer to the original acetabular edge, keeping in the limit to avoid dislocation. Small space above the cup was accepted if anterior and posterior cup edge could be supported by original bone. Then the cup was rotated until we can obtain proper screw fixation.

For the cases with severe defects, we use cup supporters and allografts. Cup supporters are designed to be bent and fit to the pelvis during the surgery. But to shape it a properly; for good coverage and strong support; is very difficult and takes long through the limited window with fatty gloves. And mean while we get more bleeding. The geometries were obtained by CAT scan of the devices. Then proper size was determined as cup size. Chemiwood model was made and proper size supporter was opened and bent preoperatively using the model. It was scanned again and compared to the pelvic geometry again.

Using cluster cups, no dangerous screw was found as long as normal cup orientation was decided and screws were less than 30mm. Posterior screws were often too short then rotated anterior and found to have good fixation. Pre-bending could reduce surgical time remarkably.

As long as we could know, no navigation system can control the cup rotation. But acetabular preoperative planning was very useful and could reduce operative invasion. It could be done easily without using navigation system.


Orthopaedic Proceedings
Vol. 91-B, Issue SUPP_I | Pages 43 - 43
1 Mar 2009
Biant L Bruce W Walker P Herrmann S Walsh W
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Intro: Total knee replacement (TKR) manufacturers offer the option of high flexion tibial tray inserts. The polyethylene is narrower posteriorly than the standard insert and comes with the theoretical risk of reduced longevity due to thinner implant. This trial studied both the intra-operative and post-operative difference in knee flexion.

Method: 100 consecutive patients undergoing posterior stabilized Genesis II TKR with Brainlab navigation were studied. The flexion of each knee was assessed per-operatively using the Brainlab navigation system for both the high flex and standard tibial insert trials. Patients were then randomized to receive either a high flex or standard definitive implant and the knee flexion measured clinically with a goniometer six months post op.

Results: Intra-operatively the high flexion insert trial flexed more than the standard flex option by 3.2 degrees. Clinically at six months post op there was no difference in knee flexion between the two groups.

Conclusion: There is no clinical difference in knee flex-ion at six months post-op in patients with a high flex or standard tibial insert. However, we believe that in certain technical circumstances the high flex option is a valuable option for the surgeon to have available.


Orthopaedic Proceedings
Vol. 88-B, Issue SUPP_III | Pages 365 - 365
1 Oct 2006
Shoeb M Coathup M Witt J Walker P Blunn G
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Introduction: Conservative hip replacements are advantageous because resection of bone in the proximal femur is minimised. This study investigated a new design of conservative hip in the goat model where the femoral head was resected and two hydroxyapatite coated ‘pegs’ were introduced into the femoral neck. The hypothesis was that the ‘pegs’ would provide a direct method of transmitting forces within the femoral neck thus resulting in less adverse bone remodelling and reduced loosening. Bone stock is also preserved should subsequent revision be required.

Methods: Eight unilateral implants were inserted into the right femur of adult female goats for 1 year. Retrieved specimens were analysed radiographically and histologically. Image analysis was used to quantify bone attachment and total bone area adjacent to the implant. Tetracycline bone markers quantified bone turnover. Operated hips were compared with non-operated hips. The students t-test was used for comparative statistical analysis where p< 0.05 were classified as significant.

Results: Radiographic analysis demonstrated bone loss beneath the cup with increased bone density at the distal end of the pins (fig.1). Light microscopy revealed areas of new and mature bone adjacent to the implant. Osseointegration to the HA coating was observed. Bone markers established significantly decreased bone formation rates (p< 0.05) in bone adjacent to the implant in the operated versus control hips.

Image analysis results demonstrated an average bone attachment of 30.94% to the implant surface (fig 2). Greatest bone attachment occurred at the end of the pins (78.99%) contributing 22% of overall attachment to the implant. Least attachment occurred beneath the prosthetic cup (13.82%) and in the medial aspect adjacent to the central pin. Greater total bone area was measured in control hips and no significant correlation between bone attachment to the ‘pegs’ and bone area beneath the prosthetic cup was identified.

Discussion: From this study we have concluded that despite the resorption of bone beneath the prosthetic cup, the conservatve hip design investigated remained well fixed in the femur during the 1 year in vivo period. It appears that an implant design that resurfaces the femoral head with two pins used to transmit forces into the femoral neck is a useful approach in conservative hip design.


Orthopaedic Proceedings
Vol. 88-B, Issue SUPP_I | Pages 88 - 89
1 Mar 2006
Auld J Langdown A Van der Wall H Walsh W Walker P Bruce W
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Background: The Profix Total Knee Arthroplasty (Smith and Nephew, Memphis, USA) is designed to replace less bone than is resected from the posterior femoral condyles, and as a consequence the posterior condylar offset is reduced. The net effect of this is to increase the flexion gap with no effect on the extension gap. This is a deliberate design philosophy aimed at increasing postoperative flexion. This prospective cohort study has tested this theory.

Methods: 60 patients underwent primary posterior cruciate retaining (CR) TKA using this prosthesis. A matched group of patients, employing a different CR prosthesis which replaces excised bone in full, served as historical controls. Intra-operative measurements were made of the posterior condylar bone resected in each case. These measurements were then correlated with the flexion achieved both intra-operatively and at 6 months post-operatively.

Results: A positive correlation between pre-operative and post-operative flexion was found. However, there was no correlation between the relative increase in flexion gap secondary to the reduction in posterior offset and the resulting flexion range.

Conclusion: Post-operative flexion range is not increased by the resection of more bone from the posterior femoral condyles than is replaced by the prosthesis in TKA. The loss of bone stock will have implications for revision surgery and should be avoided.


Orthopaedic Proceedings
Vol. 86-B, Issue SUPP_IV | Pages 425 - 425
1 Apr 2004
Haider H Walker P
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Knee simulators are now widely used for the determination of performance and wear durability of TKR’s. The International Standards Organisation (ISO) force-control option synchronises AP force and IE torque with flexion angle and axial force for the walking gait cycle. The force control concept subjects the same input waveforms to different TKRs, allowing them to move (and wear) as their designs dictate. It however relies on a mechanical spring based assembly to simulate the restraint effects of ligaments in AP and rotation. The contribution of this restraint mechanism depends on the stiffnesses of the four springs, and on how they are set at the neutral position. The springs can be loose with a gap, such that compression only starts (or ends) when the motion exceeds the gap. Alternatively the springs can be pre-compressed such that they never go loose.

A detailed mathematical model was developed which included the stiffnesses of the four springs, their settings (level of pre-compression or gap), and geometry of the mechanism to calculate a matrix of AP restraint force curves with AP displacement, and how these curves change with int-ext rotation superimposed. The same was done for rotational restraint with simultaneous linear displacement. Through an interactive computational interface, the families of curves for any combination of variables were repeatedly plotted and compared to published data on the contribution of particular ligaments to the laxity of the knee (eg. Fukubayashi et al. 1982) to find the optimum spring stiffnesses and gap configuration. This was done for simulation arrangements retaining ACL, PCL or both retained or resected. The results showed the behaviour of the system to be as sensitive to the gap and level of pre-compression, as to the stiffnesses of the springs. For the resected ACL retained PCL situation, the optimum we recommend is soft (7.24 N/mm) springs on the ACL side, harder (33.8 N/mm) springs on the PCL side, with a 2.5 mm gap on each side. For both ACL and PCL resected, the soft (7.24 N/mm) springs for both sides are optimum, again with a 2.5 mm gap on each.

These settings are obviously different from each other, and are different from the tests with this simulator published by different laboratories. The same settings are a pre-cursor for valid comparison of wear and kinematics.


Orthopaedic Proceedings
Vol. 86-B, Issue SUPP_IV | Pages 418 - 418
1 Apr 2004
Haider H Walker P
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Besides the numerous variations of TKR designs addressing fixation, wear, or specific indications, there are variations from competing design philosophies such as conformity and shape of the articulating surfaces and mobile versus fixed bearing designs. With the same resected ACL and retained PCL ligament combinations and similar surgical procedure, the subset of different implants for these very indications should be expected to produce only minor variations in kinematics. This study set out on a comprehensive series of detailed and intricately controlled in-vitro tests to examine this hypothesis. Six different posterior cruciate retaining medium size knees from different manufacturers were used. Four were fixed bearing condylar types of low to high constraint; and two mobile bearing ones which allowed rotational and translational freedom, one fully and one partially conforming. The implants were aligned according to the manufacturer’s recommendations and subjected to the same ISO force-control simulation. The kinematics captured from the averaged simulated cycles of walking showed AP displacement contained within an envelope of 4 mm for most of the stance phase. This increased with most to a maximum range of 5mm just before toe-off at the end of the stance phase. In rotation, the designs showed ranges during stance from about 2–13 degrees. The kinematics from the different implant designs were thus significantly different; a controversial answer regarding the hypothesis posed. This means the “performance” must be different between these implants if installed “ideally” on the same patient with the PCL retained. Studies are worthwhile to determine if these differences in performance are reflected in clinical functional conditions.


Orthopaedic Proceedings
Vol. 85-B, Issue SUPP_I | Pages 14 - 14
1 Jan 2003
Ahir S Bayley J Walker P Squire-Taylor C Blunn G
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The restoration of pain-free stable function in gleno-humeral arthritic cases in various situations such as rotator cuff deficiency, old trauma and failed total shoulder arthroplasty is a challenging clinical dilemma. The Bayley-Walker shoulder has been designed specifically for very difficult cases where surface replacement devices do not provide sufficient stability. This device is a fixed-fulcrum reversed anatomy prosthesis consisting of a titanium glenoid component with a CoCrMo alloy head that articulates with an UHMWPE liner encased in a titanium alloy humeral component that has a long tapered grooved stem. The centre of rotation of the Bayley-Walker shoulder is placed medially and distally with respect to the normal shoulder in order to improve the efficiency of the abductor muscles. An important problem in devices of this type is obtaining secure and long-lasting fixation of the glenoid component. The glenoid component relies on fixation through the cortical bone by using threads, which protrude through the anterior surface of the scapula at the vault of the glenoid. It is HA coated for subsequent osseointegration. The purpose of this study was to investigate fixation of the glenoid component.

A 3D finite element model of the glenoid component implanted in a scapula was analysed using Abaqus. The implant was placed in position in the scapula, with the final 2–3 screw threads cutting through the cortical bone on the anterior side at the vault of the glenoid due to the anatomy in this region. The analysis was performed for two load cases at 60° and 90° abduction. A histological study of a retrieval case, obtained 121 days after implantation, was also conducted.

The FEA results showed that most of the forces were transmitted from the component to the cortical bone of the scapula, the remaining load being transmitted through cancellous bone. In particular the area where the threads of the glenoid component penetrated the scapula showed high strain energy densities. Histology from the retrieved case showed evidence of bone remodelling whereby new bone growth resulting in cortical remodelling had occurred around the threads.

Both the FEA and histological study show that fixing the component at multiple locations in cortical bone may overcome the problems of glenoid loosening associated with constrained devices. The Bayley-Walker device has been used on a custom basis since 1994; 81 Bayley–Walker shoulders for non-tumour conditions and 43 Bayley-Walker glenoid components have been used in association with a bone tumour implant, with good early results. Radiographically, radiolucencies have not been observed and overall the comparisons with the original Kessel design are positive.


The Journal of Bone & Joint Surgery British Volume
Vol. 77-B, Issue 5 | Pages 705 - 714
1 Sep 1995
Walker P Mai S Cobb A Bentley G Hua J

We report the theoretical basis of a method to measure axial migration of femoral components of total hip replacements (THR). The use of the top of the greater trochanter and a lateral point on the collar of the stem, allowing for variations of up to 10 degrees rotation of the femur in any direction between successive radiographs, gave a maximum error of 0.37 mm. At a more realistic 5 degrees rotational variation, the error was only 0.13 mm. These data were confirmed in an experimental study using digitisation of points and special software. We also showed that the centre of the femoral head, the stem tip, and the lesser trochanter provided less accurate landmarks. In a second study we digitised a series of radiographs of 51 Charnley and 57 Stanmore THRs; the mean migration rates were found to be identical. We then studied 46 successful stems with a minimum follow-up of eight years and 46 stems which had failed by aseptic loosening at different times. At two years, the successful stems had migrated by a mean of 1.45 +/- 0.68 mm, but the failed cases had a mean migration of 4.32 +/- 2.58 mm (p < 0.0001). Of the successful cases 76% had migrated less than 2 mm, while in the failed group 84% had migrated more than 2 mm. For any particular case migration of more than 2.6 mm at two years had only a 5% chance of continuing success and would therefore merit special follow-up. Only 24% of the eventually successful stems showed migration at the stem-cement interface, but this had happened in every failed stem. We conclude that it would be possible to evaluate a new cemented design of femoral stem over a two-year period by the use of our method and to compare its performance against the reported known standard of the Charnley and Stanmore designs.


The Journal of Bone & Joint Surgery British Volume
Vol. 73-B, Issue 5 | Pages 757 - 761
1 Sep 1991
Inglis A Walker P

Forty failed hinged arthroplasties of the knee were revised by the insertion of another hinged implant. In 14 cases the prosthesis used at the revision operation was similar to the primary implant; in 26, a hinge with an elongated femoral stem was used, usually replacing part of the femoral shaft. In seven of these knees an elongated tibial stem was also required, though the tibial shaft was replaced in only two of them. There were many complications. Fracture of the femur at the tip of the femoral stem was the most frequent. Sixteen first revisions failed and were revised a second time; 12 required replacement of the distal femoral shaft and three required replacement of the proximal tibia. The incidence of complications in knees requiring a second revision was even higher. Four required a third revision after an average interval of three years. Failure of a hinged prosthesis results in bone loss mainly in the femur. Revision of a failed hinged prosthesis with another of the same design is unlikely to be successful and may cause fracture of the femur.


The Journal of Bone & Joint Surgery British Volume
Vol. 59-B, Issue 2 | Pages 222 - 228
1 May 1977
Walker P Hsieh H

Experiments were carried out to determine the optimum conformity between the femoral and tibial condyles in condylar replacement knee prostheses. Wear tests and observations from removed prostheses indicated that both high and low conformity produced characteristic abrasion and fatigue. Partly conforming condyles provided stability under load-bearing but allowed laxity to occur. Fixation to resist the various forces on the tibial components was enhanced by a short central intramedullary peg. Partial conformity is proposed as the optimum configuration between femoral and tibial components.