Objective: To determine the usefulness of computed tomograpy (CT) scans in the management of ankle fractures in children.
to measure the total metal content in cell saver blood recovered during revision hip arthroplasty, to evaluate the efficacy of centrifuging and washing the recovered blood in reducing the metal content, to investigate whether transfusion of the salvaged blood resulted in a significant increase in the metal ion levels in the patients’ blood in the immediate post-operative period.
We report 17 patients (20 hips) in whom metal-on-metal resurfacing had been performed and who presented with various symptoms and a soft-tissue mass which we termed a pseudotumour. Each patient underwent plain radiography and in some, CT, MRI and ultrasonography were also performed. In addition, histological examination of available samples was undertaken. All the patients were women and their presentation was variable. The most common symptom was discomfort in the region of the hip. Other symptoms included spontaneous dislocation, nerve palsy, a noticeable mass or a rash. The common histological features were extensive necrosis and lymphocytic infiltration. To date, 13 of the 20 hips have required revision to a conventional hip replacement. Two are awaiting revision. We estimate that approximately 1% of patients who have a metal-on-metal resurfacing develop a pseudotumour within five years. The cause is unknown and is probably multifactorial. There may be a toxic reaction to an excess of particulate metal wear debris or a hypersensitivity reaction to a normal amount of metal debris. We are concerned that with time the incidence of these pseudotumours may increase. Further investigation is required to define their cause.
We report the clinical, radiographic and histologic features of these cases.
There were no infections in this series. Three subjects required revision surgery.
This study suggests that resurfacing arthoplasty can also induce a local hypersensitivity reaction in response to metal wear debris. It therefore raises new concerns regarding the long-term safety of this procedure.
The creep and wear behaviour of highly cross-linked polyethylene and standard polyethylene liners were examined in a prospective, double-blind randomised, controlled trial using radiostereometric analysis. We randomised 54 patients to receive hip replacements with either highly cross-linked polyethylene or standard liners and determined the three-dimensional penetration of the liners over three years. After three years the mean total penetration was 0.35 mm (SD 0.14) for the highly cross-linked polyethylene group and 0.45 mm (SD 0.19) for the standard group. The difference was statistically significant (p = 0.0184). From the pattern of penetration it was possible to discriminate creep from wear. Most (95%) of the creep occurred within six months of implantation and nearly all within the first year. There was no difference in the mean degree of creep between the two types of polyethylene (highly cross-linked polyethylene 0.26 mm, SD 0.17; standard 0.27 mm, SD 0.2; p = 0.83). There was, however, a significant difference (p = 0.012) in the mean wear rate (highly cross-linked polyethylene 0.03 mm/yr, SD 0.06; standard 0.07 mm/yr, SD 0.05). Creep and wear occurred in significantly different directions (p = 0.01); creep was predominantly proximal whereas wear was anterior, proximal and medial. We conclude that penetration in the first six months is creep-dominated, but after one year virtually all penetration is due to wear. Highly cross-linked polyethylene has a 60% lower rate of wear than standard polyethylene and therefore will probably perform better in the long term.
The Charnley Elite femoral component was first introduced in 1992 as a new design variant of the original Charnley femoral component (De Puy, Leeds, UK) with modified neck and stem geometry. The original component had undergone few changes in nearly forty years and has excellent long-term results. Early migration of the new stem design was determined by Roentgen Stereophotogrammetric Analysis (RSA)1. Rapid early migration of a component relative to the bone, measured by RSA, is predictive of subsequent aseptic loosening for a number of femoral stems. As there was rapid early migration and rotation of the Charnley Elite stem, we predicted that the long-term results would be poor. An outcome assessment is indicated as stems of this type are still being implanted. One hundred Charnley Elite stems, implanted in our centre between 1994 and 1997 were included in a prospective, cross-sectional follow-up study. Outcome measures include validated clinical scores (Charnley hip score, Harris hip score and Oxford hip score) and radiological scores (Gruen classification) as well as revision rates over the past 10 years. The clinical follow-up supports the RSA predictions of early failure of the Charnley Elite femoral stem.
Patients had improvement of pain, posture, hip instability, walking ability and limb length discrepancy. The median lengthening of the femur was 3 cm (2–5). The mechanical axis was realigned in all patients. All patients were satisfied with the outcome. Planned secondary contra lateral epiphyseodesis was required to equalise leg length in 2 patients. Complications included a stiff knee (1) that required a Judet quadricepsplasty, premature consolidation (1) that required reosteotomy and knee subluxation (1) that required cross knee stabilisation.
We changed our pin tract care practice from 1996. We had a significant decrease in pin tract infections since then (p<
0.0001). We also found that using Ilizarov wires had significantly less infections than with half pins used with monolateral fixators (p<
0.0001; linear trend, p= 0.0338). There were 48 patients that required hospital admissions for IV antibiotics. and of these 10 patients required debridement. There were no residual long lasting infections or chronic osteomyelitis.
Arthroscopic procedures may be associated with considerable pain in the first 24 hours. Intra-articular bupi-vacaine provides good analgesia but is short lasting. Intra-articular morphine has been shown to prolong postoperative analgesia in knee and ankle arthroscopy. The aim of this study is to assess the safety and analgesic effect of intra-articular morphine following day case wrist arthroscopy. Ethical approval was firstly obtained. 31 patients were randomly assigned to one of 2 groups in a double blind clinical study. Group 1 received 5ml of 0.5% bupi-vacaine intra-articularly with 5mg of morphine subcutaneously. Group 2 received 5ml of 0.5% bupivacaine and 5mg of morphine intra-articularly. There were 15 patients (mean age 41.2 years) in group 1, and 16 patients (mean age 38.9 years) in group 2. Postoperatively pain was assessed using a 100mm visual analogue pain scale (VAPS) at 1, 2, 6 and 24 hours. Analgesia requirements were recorded at these times post operatively. The presence of nausea, vomiting, other complications and patient satisfaction were recorded. Visual analogue pain scores did not show any significant difference between the groups at 1, 2, 6 and 24 hours. Supplementary analgesic consumption over the 24 hour period was slightly greater in group 1 than in group 2. None of the patients who had intra-articular Morphine had vomiting nor any other complications and did not require anti-emetics. Most patients in either group were satisfied with the level of postoperative analgesia. Intra-articular bupivacaine with or with out morphine provides adequate postoperative pain relief following wrist arthroscopy. There seems to be little difference between the two methods studied.
Early migration of the new stem design was determined by Roentgen Stereophotogrammetric Analysis (RSA). Rapid early migration of a component relative to the bone, measured by RSA, is predictive of subsequent aseptic loosening for a number of femoral stems. As there was rapid early migration and rotation of the Charnley Elite stem, we predicted that the long-term results would be poor. An outcome assessment is required as stems of this type are still being implanted.
Preliminary clinical scores in the patients who had not undergone any subsequent surgery were adequate (Oxford Hip Score mean average of 23.9). Thirteen percent of radiographs analysed had evidence of loosening, giving an overall loosening rate of 14% at 8 years.
We performed a clinical and radiological study to determine the rate of failure of the Charnley Elite-Plus femoral component. Our aim was to confirm or refute the predictions of a previous roentgen stereophotogrammetric analysis study in which 20% of the Charnley Elite-Plus stems had shown rapid posterior head migration. It was predicted that this device would have a high early rate of failure. We examined 118 patients at a mean of nine years after hip replacement, including the 19 patients from the original roentgen stereophotogrammetric study. The number of revision procedures was recorded and clinical and radiological examinations were performed. The rate of survival of the femoral stems at ten years was 83% when revision alone was considered to be a failure. It decreased to 59% when a radiologically loose stem was also considered to be a failure. All the patients previously shown in the roentgen stereophotogrammetric study to have high posterior head migration went on to failure. There was a highly significant difference (p = 0.002) in posterior head migration measured at two years after operation between failed and non-failed femoral stems, but there was no significant difference in subsidence between these two groups. Our study has shown that the Charnley Elite-Plus femoral component has an unacceptably high rate of failure. It confirms that early evaluation of new components is important and that roentgen stereophotogrammetric is a good tool for this. Our findings have also shown that rapid posterior head migration is predictive of premature loosening and a better predictor than subsidence.
This study presents the 2 year migration results of the BHR femoral component using Roentgen Stereophoto-grammteric Analysis (RSA).
Polished, tapered stems are now widely used for cemented total hip replacement and many such designs have been introduced. However, a change in stem geometry may have a profound influence on stability. Stems with a wide, rectangular proximal section may be more stable than those which are narrower proximally. We examined the influence of proximal geometry on stability by comparing the two-year migration of the Exeter stem with a more recent design, the CPS-Plus, which has a wider shoulder and a more rectangular cross-section. The hypothesis was that these design features would increase rotational stability. Both stems subsided approximately 1 mm relative to the femur during the first two years after implantation. The Exeter stem was found to rotate into valgus (mean 0.2°,
We prospectively reviewed 24 patients (35 feet) who had been treated by a Scarf osteotomy and Akin closing-wedge osteotomy for hallux valgus between June 2000 and June 2002. There were three men and 21 women with a mean age of 46 years at the time of surgery. The mean follow-up time was 20 months. Our results showed that 50% of the patients were very satisfied, 42% were satisfied, and 8% were not satisfied. The mean American Orthopaedic Foot and Ankle Society score improved significantly from 52 points pre-operatively to 89 at follow-up (p <
0.001). The intermetatarsal and hallux valgus angles improved from the mean pre-operative values of 15° and 33° to 9° and 14°, respectively. These improvements were significant (p <
0.0001). The change in the distal metatarsal articular angle was not significant (p = 0.18). There was no significant change in the mean pedobarographic measurements of the first and second metatarsals after surgery (p = 0.2). The mean pedobarographic measurements of the first and second metatarsals at more than one year after surgery were within the normal range. Two patients had wound infections which settled after the administration of antibiotics. One patient had an intra-operative fracture of the first metatarsal and one required further surgery to remove a long distal screw which was irritating the medial sesamoids. We conclude that the Scarf osteotomy combined with the Akin closing-wedge osteotomy is safe and effective for the treatment of hallux valgus.
The treatment of acetabular dysplasia in adolescents (age>
12) is difþcult and various complex pelvic osteotomies have been described. The aim of surgery being improvement in pain and to delay the onset of secondary osteoarthrosis.
Increasing the width of the proximal section of a polished tapered stem enhances its rotational stability.
There were 3 male and 21 female patients with a mean age of 46 years. The mean follow up was 17 months. Statistical analysis was carried out.
The Birmingham hip resurfacing (BHR) arthroplasty is a metal-on-metal prosthesis for which no medium- or long-term results have been published. Despite this, it is increasing in popularity as an alternative to stemmed prostheses for younger patients. Since the fixation of the socket is conventional, the major concern is long-term failure of the femoral component. This can be predicted by the use of roentgen stereophotogrammetric analysis (RSA). We have therefore undertaken such a study of the BHR femoral component over a period of two years. Twenty patients (22 hips) underwent a standard BHR procedure. Migration of the femoral component was measured by RSA at intervals of three, six, 12 and 24 months. At 24 months the total three-dimensional migration of the head was 0.2 mm. This was not statistically significant. Previous studies have shown that implants which loosen quickly have rapid early migration. Our results therefore suggest that the BHR femoral component is an inherently stable device which is likely to perform well in the long term.
The use of exhaust suit systems is commonplace in arthroplasty surgery where isolation of the surgical team is desirable in an attempt to reduce the risk of infection transmission. Elevated carbon dioxide levels have been reported in the non-clinical setting with such systems the consequences of which can include fatigue, diapho-resis, nausea, headache and irritability. The aim of our study was to determine the levels of carbon dioxide present within an exhaust suit system during hip arthroplasty and to compare these with the recommended occupational exposure limit levels published by the Health and Safety Executive (HSE). Data was collected during ten primary hip replacements performed by the same surgeon whilst wearing the Stryker Steri-Shield Helmet Exhaust System. This is a self-contained unit with an integrated blower used in conjunction with a full-length gown. In addition the helmet was fitted with an air-sampling probe connected to a portable infrared CO2 monitor and also a temperature probe. Thus continuous monitoring of both CO2 and temperature level during surgery was possible. The mean initial CO2 concentration in the helmet at the beginning of surgery was 3 000 parts per million (ppm) and the mean maximum CO2 level recorded was 13 000 ppm. The mean time the surgeon was within an exhaust suit to perform a primary hip replacement was 1 hr 54 mins and for 86% of this time period the CO2 level within the helmet exceeded the recommended level of 5 000 ppm as stipulated by the HSE. In conclusion we have demonstrated significantly elevated CO2 levels within the Stryker Steri-shield Exhaust Suit System during hip surgery. Surgeons who use this system should be aware of this together with the physical symptoms that may result.
The relative motion between a prosthesis, the cement mantle and its’ host bone during weight bearing is not well understood. Using Radiostereophotogrammetric Analysis (RSA), we examined the dynamically inducible micromotion that exists at these interfaces when an increased load is placed through the prosthesis. Dynamically inducible micromotion was measured in the femoral components of 21 subjects undergoing total hip replacement with polished Exeter stems. Two standing RSA studies were performed, at 3 and 12 months postoperatively. Firstly in double-leg stance, and secondly fully weight bearing through the operated hip. Subjects had no signs of clinical or radiological signs of loosening at 1 year. Significant micromotion was detected at the prosthesis-cement interface at 3 months. Similar patterns of micromotion were observed at 12 months. The prosthesis appeared to bend during single-leg stance weight bearing, however this accounted for less than half of the total observed movement. Conventional RSA studies were conducted at 3 months, 6 months and 1 year to confirm that the implants showed normal migration patterns. This study demonstrates that movement exists between the prosthesis and bone during cyclical weight bearing. This dynamically inducible micromotion probably occurs at the prosthesis-cement interface. It could account for the wear that is observed on the surface of retrieved secure prostheses. This may be a mechanism by which failure eventually occurs.
The Birmingham reSurfacing Arthroplasty (BSA) is a metal on metal prosthesis with no published independent clinical studies. Despite this, it is increasing in popularity, especially as an alternative to stemmed prostheses in younger patients. This study presents the 1year migration results of the BSA femoral component using Roentgen Stereophotogrammteric Analysis (RSA). Twenty six subjects underwent a BSA, through the postero-lateral approach using CMW3G cement, with RSA marker balls placed intra-operatively. The femoral component migration was measured at intervals of 3, 6 and 12 months using the Oxford RSA system. Geometric algorhythms were used to identify the femoral component. The data was examined for distribution prior to analysis. All statistical analysis was performed using the t-test. The data was normally distributed. The 1 year migration results of the BSA femoral component are displayed below. All cemented implants migrate in vivo. The majority of cemented stemmed implant migration occurs within the first post-operative year. High rates of migration within the first post-operative year correlate with premature component failure in some instances. The BSA is a fundamentally different design to most cemented prostheses, despite this we know that very large migrations, those in excess of 2mm/year in any direction are generally regarded as poor indicators of long term outcome. These results suggest that the BSA femoral component is an inherently stable device as it does not migrate significantly within the first post-operative year. Only long-term independent clinical studies and continued RSA follow-up will enable a comprehensive evaluation of the device.
We have treated seven children with relapsed infantile Blount’s disease by elevation of the hemiplateau using the Ilizarov frame. Three boys and four girls with a mean age of 10.5 years were reviewed at a mean of 29 months after surgery. All had improved considerably and were pleased with the results. The improvements in radiological measurements were statistically significant (p <
0.001). Three-dimensional CT reconstruction was useful for planning surgery. There were no major complications. The advantages of this technique are that in addition to elevation of the hemiplateau, rotational deformities and limb-length discrepancies may be addressed.
We review the results of a modified quadricepsplasty in five children who developed stiffness of the knee after femoral lengthening for congenital short femur using an Ilizarov external fixator which spanned the knee. All had a full range of movement of the knee before lengthening was undertaken. Unifocal lengthening was carried out in the distal metaphysiodiaphyseal region of the distal femur with a mean gain of 6.5 cm. The mean percentage lengthening was 24%. At the end of one year after removal of the Ilizarov frame and despite intensive physiotherapy all patients had stiffness. Physiotherapy was continued after the quadricepsplasty and, at the latest follow-up (mean 27 months), the mean active flexion was 102° (80 to 130). The gain in movement ranged from 50° to 100°. One patient had a superficial wound infection which settled after a course of oral antibiotics. None developed an increased extension lag after surgery and all were very satisfied with the results. Quadricepsplasty is a useful procedure for stiffness of the knee after femoral lengthening which has not responded to physiotherapy.
Reflex sympathetic dystrophy is a syndrome characterised by pain and hyperaesthesia associated with swelling, vasomotor instability and dystrophic changes of the skin. It is rare in children, can occur without any previous history of significant trauma and may be recurrent and migratory. We reported 13 new cases of RSD in children and emphasised the role of a multidisciplinary team approach in management. A review of the literature was included. 13 children (3 boys and 10 girls) with reflex sympathetic dystrophy were presented. They were aged between 8 and 17 years. Mean age at onset was 13 years 4 months. All of them had RSD involving the lower limbs. Thermography was performed in 10 cases. The average time to correct diagnosis was 4 months. Five ankles, 4 knees and 5 hips were involved (14 joints in 13 cases). Psychological assessments revealed abnormalities in all cases. Pain (visual analogue score) and function were assessed before and after treatment. The most common therapy in children is progressive mobilisation supported by analgesic drugs, psychological and physical therapy. We individualised the therapy for each child. A team-care approach with the physiotherapist, psychologist and pain-care team co-ordinated by the Orthopaedic Consultant was the essence of our management. All children received physical therapy including a wide variety of non-standarised approaches involving analgesics and hydrotherapy. 5 patients received guanithidine blocks. Individual therapy was monitored with set achievable goals and weekly assessment of progression of mobility and joint motion. Time from the first RSD episode to resolution averaged 6 months in our series [it was mean 10 weeks in the non-adolescent cases (8 cases) and 7 months in the adolescent one (5 cases)]. The pain and function scores improved remarkably in all patients. RSD in children is not a widely recognised condition. There is often a delay in diagnosis in view of the rarity of the condition as well as the fact that specific diagnostic modalities are not readily available in all centres. Psychological factors should not be underestimated. Early diagnosis with an aggressive, multidisciplinary, monitored, ‘goal-oriented’ team approach should be the basis of management in these cases.
This study was designed to assess the standard of orthopaedic training of Senior House Officers in the U.K. and to determine the optimum time that should be spent in these posts before registrar training. Two MCQ papers were constructed. One for the pre test and one for the post test. Questions covered all aspects of orthopaedics and trauma including operative surgery. The paper was firstly tested on controls including medical students, house officers, registrars of various grades and consultants. There was no statistical difference in the results for the two papers within the groups indicating that pre and post test papers were of similar standard. In addition the average scores in the tests increased proportionately to the experience and grade of the control. 129 SHOs from 25 hospitals in 10 different regions were tested by MCQ examination at the beginning of their 6-month post. They were again tested at the end of the job. The differences in score were compared. This difference was then correlated with the experience and career intention of the SHO. There was no statistical difference between pre and post test results in all groups of SHOs in the study (student t test). The best improvement in scores during this six month period were seen in SHOs of 1–1.5 years orthopaedic experience. SHOs of more than 3 years experience demonstrated the smallest improvement in their score. There was a net loss of seven trainees with a career intention of orthopaedics to other disciplines. In the vast majority of Senior House Officer posts in this country, very little seems to be learnt during a six-month attachment. This is especially so for those who are doing orthopaedics for the first time as well as very experienced SHOs.
To determine whether resection of osteophyte at TKR improves movement, 139 TKRs were performed on knees with pre-operative posterior osteophyte. Randomisation was to have either resection of distal femoral osteophyte guided by a custom made ruler or no resection. After preparation of the femoral bone cuts the ruler measuring 19 mm was placed just proximal to the posterior chamfer cut. The proximal end of this ruler marked the bone to be resected and this was performed using an osteotome at 45 degrees. Knees randomised to no resection had no further femoral bony cuts. Three months after implantation the patients had range of motion assessed. One hundred and fourteen suitable knees were assessed, with 59 knees (57 patients) in the resection group and 55 knees (54 patients) in the no resection group. Full extension was more likely in the resection group (62%) than the group without resection (41%)(p=0.08). Flexion to at least 110 degrees was, however, less in the resection group (37%) than the no resection group (54%) (p=0.09). Our study failed to show a statistically significant difference if the bony osteophyte is removed. There were however sharp trends, with statistically a one in ten chance these results would be different if the trial was repeated. Although there is no indication as to the cause of improved extension this could be explained by the release of the posterior capsular structures allowing full extension. The reduction in flexion is harder to explain and this may be due to increase in perioperative trauma and resultant swelling, possibly with fibrosis. Range of movement, particularly flexion, is known to improve up to 1 year post-operatively and assessment of these groups at that stage would be beneficial.
We present the results of treatment for relapsed infantile Blounts disease using a technique of hemiplateau elevation with the Ilizarov frame. 7 patients with a mean age of 10 years 6 months were reviewed at 30 months following hemiplateau elevation with/without ipsilateral tibial lengthening. Preoperatively clinical photographs, long leg standing radiographs and 3D computed tomography images were acquired. The patients were evaluated clinically (presence of knee pain, range of knee motion, knee stability and leg length discrepancy) and radiologically. Schoeneckers objective assessment was undertaken. The results were analysed statistically. Clinically all the patients improved significantly. No patient had knee pain and the range of knee motion was from 0° to more than 100° of knee flexion. The radiological results and Schoeneckers grading are depicted below. The improvement in radiological measurements were statistically significant. Pin site infection was present in all but settled with antibiotics. The results of hemiplateau elevation for relapsed infantile Blounts disease are encouraging.
This study examined the effect of surgical approach on the 1year migration of the Exeter stem, using Roentgen Stereophotogrammetric Analysis (RSA). There is evidence that implants with increased early migration, particularly those moving into internal rotation, are likely to have a higher failure rate. A total of 46 patients awaiting THR were allocated into 2 groups. Both groups underwent an Exeter Total Hip replacement, one through the Posterior Approach (PA), and the other through the Hardinge Approach (HA). RSA was used to assess post-operative stem position and migration at 1 year. Post-operative gait analysis and clinical evaluation was used to assess a subgroup of patients. There was no difference in the initial stem position of each group within the femoral canal. Significant differences in migration were found for migration of the head distally and posteriorly and for the tip medially. During gait, the legs of the PA group were relatively internally rotated when compared to those of the HA group. Relative to the supporting bone, the PA implants internally rotate and also rotate into valgus further than the HA implants. The starting position of the prostheses was the same and the cement and prosthesis characteristics were similar. From this, we can infer that surgical approach and therefore muscle function, have a large influence on component migration. During gait, the PA group had internally rotated legs compared to the HA group. This is probably due to short external rotator weakness in the PA group. As a result the internally rotating component of the posteriorly directed joint reaction force on the femoral stem will be larger. This probably explains the rapid internal rotation seen in the implants of the PA group. With the HA group, because of abductor damage there is likely to be a smaller component of the joint reaction force acting on the stem. This would explain the smaller rates of observed coronal plain rotation (rotation into valgus). Profound differences exist in early migration, between the HA and PA. Different muscle function may account for this and probably influences long-term outcome.
This early study examines the influence of a wider shoulder on the 1 year migration of a cemented, polished, tapered stem, using RSA. Polished, tapered stems (PTS) have excellent 10 year survival rates. RSA studies have demonstrated that these devices subside about 1 mm / year. Small amounts of subsidence are beneficial in stabilising a stem. Stem rotation (measured as posterior head migration) within the cement mantle is probably a more important mechanism of failure than subsidence. Stems with a wider proximal portion are thought to better resist rotation. The CPS (Endoplus, UK) is such a device; here we compare its’ stability with that of the Exeter. 20 patients received the CPS-plus stem and underwent RSA examinations at 3, 6 and 12 months postoperatively. The Exeter 1 year migration data was used as a comparison. Both groups underwent a Hardinge approach with CMW3G cement. Both stems subsided about 1mm. The CPS showed less medio-lateral and A-P movement of the proximal stem than the Exeter over 1 year, as shown below: The CPS internally rotates less than the Exeter, as demonstrated by the smaller amount of posterior head migration. It has a lateral flare of the shoulder; making its cross-section wider than the Exeter’s, this probably accounts for its’ greater resistance to rotation. The CPS also undergoes less medio-lateral proximal stem migration. Its’ lateral shoulder flare is probably responsible for this axial subsidence, as it prevents the shoulder from moving laterally whilst subsiding over the calcar. A PTS with a broad proximal section is more stable, this may confer an increased survival advantage.
A significant difference was noted both for the Lachmanns (p<
0.01) and pivot shift test (p<
0.001) between the results obtained with and without anaesthesia. This however did not change the management.
Isolated wear of the polyethylene tibial component led to failure in five of a series of 108 uncemented porous-coated knee replacements. The clinical features included pain, effusion and instability with progressive varus deformity. In all cases there was extensive wear on the medial side of the polyethylene surface of the prosthesis. The mechanism of such wear is complex, being due in part to the unconstrained nature of the joint and the incongruity of its surfaces. Other design characteristics may have contributed.
Since 1981, during operations for spinal deformity, we have routinely used electrophysiological monitoring of the spinal cord by the epidural measurement of somatosensory evoked potentials (SEPs) in response to stimulation of the posterior tibial nerve. We present the results in 1168 consecutive cases. Decreases in SEP amplitude of more than 50% occurred in 119 patients, of whom 32 had clinically detectable neurological changes postoperatively. In 35 cases the SEP amplitude was rapidly restored, either spontaneously or by repositioning of the recording electrode; they had no postoperative neurological changes. One patient had delayed onset of postoperative symptoms referrable to nerve root lesions without evidence of spinal cord involvement, but there were no false negative cases of intra-operative spinal cord damage. In 52 patients persistent, significant, SEP changes were noted without clinically detectable neurological sequelae. None of the many cases which showed falls in SEP amplitude of less than 50% experienced neurological problems. Neuromuscular scoliosis, the use of sublaminar wires, the magnitude of SEP decrement, and a limited or absent intra-operative recovery of SEP amplitude were identified as factors which increased the risk of postoperative neurological deficit.