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Biomedical subjects

A G Cobb

Publications and source records attributed to A G Cobb.

At least 19 recordsLinked to original sources
PubMed2006

Development rationale for an articular surface replacement: a science-based evolution.

Hip resurfacing has an enduring appeal because of the advantages of bone conservation and maximal joint stability. However, a far from satisfactory experience with earlier resurfacing designs led to its virtual disappearance in the 1980s. The concept was reintroduced in the late 1990s. The current generation of resurfacing devices generally consisted of a large-diameter metal-on-metal articulation, the femoral components being cemented and the acetabular components utilizing various forms of cementless fixation. The encouraging medium-term results, with a follow-up of up to 8 years using the current generation of surface replacement joints, combined with favourable reports related to long-term performance of some metal bearings have led to a rapid increase in the use of such components with these devices. This trend is most marked in younger, more active patients who have expectations of restoration of lifestyle in addition to improved mobility and pain relief and in whom failure with conventional total hip replacement is much higher than previously reported with more sedentary patients. The aim of this paper is, firstly, to highlight a number of areas of improvement and, secondly, to explain how these may be addressed by making modifications to the design of both implants and instrumentation and to the surgical technique. The areas identified for improvement were tissue preservation (thinner components, and reduced steps between sizes), acetabular cup issues (fixation, insertion, and positioning), femoral component issues (design, loading, and cementation), improved bearing surface characteristics, and simplified precise instrumentation with a low-trauma surgical technique.

Arthroplasty, Replacement, Hip↗
PubMed2006

The clinical significance of metal ion release from cobalt-chromium metal-on-metal hip joint arthroplasty.

Metal-on-metal (MOM) bearings offer extremely low wear and the avoidance of polyethylene but generate metallic wear particles. Although their total volume is dramatically smaller than polyethylene debris, these particles are in the nanometre size range and are many times more numerous. Metallic particles are ingested by macrophages or may be disseminated via lymphatics to the reticuloendothelial system. They corrode, and metal ions are present in the circulation and concentrated in erythrocytes. Excretion of metal ions via the kidneys seems to balance their generation in patients with MOM implants. However, highly sensitive detection methods can be used to show that levels of circulating cobalt and chromium ions are several times the normal level. These concentrations are well within the limits identified as dangerous to health in workers exposed to industrial chemicals, and also considerably lower than the levels found to cause cell toxicity in vitro. The local concentrations of particles and metal ions in the synovial tissue may occasionally exceed these limits and cause tissue necrosis. Clinical experience of lysis is rare in association with MOM bearings, as are hypersensitivity reactions and MOM bearings have had an excellent record over four decades and have a favourable benefit to risk ratio. Further reduction in risk will be achieved by improvement of materials, engineering, and accuracy of insertion.

Bone Neoplasms↗
PubMed1998

Distal femoral osteotomy for lateral compartment osteoarthritis of the knee.

Twenty-one patients with lateral compartment osteoarthritis and valgus deformity of the knee underwent distal femoral supracondylar osteotomy (medial closing wedge) between 1983 and 1993 with follow-up ranging from 1 to 8 years. Ten knees had plaster cast immobilization, 5 had fixation with 2 staples supplemented with a plaster cast, and 6 knees had rigid internal fixation with an AO blade plate. Thirty-three percent of patients had a satisfactory result using the HSS score, and 57% had a satisfactory result using the Knee Society Clinical Rating. Fifty-seven percent had a significant complication, including severe knee stiffness requiring manipulation under anesthesia (48%), nonunion/delayed union (19%), infection (10%), and fixation failure (5%). Five (19%) knees required total knee replacement within 5 years of surgery. Satisfactory results were obtained only in those patients who had less severe degrees of osteoarthritis confined to the lateral compartment (grades I to III), adequate correction of valgus deformity (the anatomical axis within 2 degrees from zero), and rigid internal fixation to permit postoperative early mobilization. These results indicate that distal femoral osteotomy is a satisfactory procedure in the young, active patient with osteoarthritis of the lateral compartment of the knee, but requires precise surgical technique and rigid internal fixation.

Adult↗
PubMed1996

Stanmore compared with Charnley total hip replacement. A prospective study of 413 arthroplasties.

From 1982 to 1987, we randomised prospectively 413 patients requiring primary total hip replacements to receive either a Stanmore or Charnley prosthesis. They were reviewed by an independent observer in an attempt to correlate a number of factors including femoral head size with longevity. There were 213 Stanmore hips and 200 Charnley prostheses. At five to ten years (mean 6.5) 76 patients had died and 16 arthroplasties had required revision. Seven were radiologically loose in asymptomatic patients. There was only one case of deep infection. We found no difference statistically in the clinical outcome or in the revision rate of 4% in the two types of prosthesis. The revision rate was greater for trainees than for senior operating surgeons, and there were recognisable technical errors in seven of the nine Stanmore, and four of the seven Charnley replacements which required revision. Retrospective radiological analysis of a random subset of 51 Charnely and 57 Stanmore femoral components showed no difference in femoral subsidence, but in 14 patients who had had bilateral replacements with one femoral component of each type, there was greater early subsidence of the Stanmore prosthesis. Our results confirm that conventional cemented total hip replacements give acceptable results in a general teaching unit, and we found no evidence of any effect of the size of the femoral head on wear or loosening at five to ten years.

Adolescent↗
PubMed1996

Hypersensitivity in aseptic loosening of total hip replacements. The role of constituents of bone cement.

Aseptic loosening is seen in a significant proportion of cemented total hip replacements (THR). In a small subgroup of patients who suffer early loosening polyethylene debris is unlikely to be responsible. We recently reported one case of allergic contact dermatitis to N,N-dimethylparatoluidine (DMT), an accelerator used in bone cement. We have therefore investigated this using skin-patch tests to a variety of substances including metals, polyethylene and the separated individual components of Simplex cement. We studied 70 patients, 15 with aseptic loosening less than two years after THR, 25 with satisfactory long-term cemented fixation, five with infected loosening of cemented THRs and 25 awaiting hip arthroplasty. We found seven positive reactions to DMT, all of them in patients with the rapid onset of aseptic loosening. Allergy to DMT is recognized in the dental profession in respect of the 'denture sore mouth' syndrome, and could also be an occupational hazard since some industrial glues contain DMT. Our results suggest the need for awareness of possible previous dental or occupational exposure of the constituents of bone cement. We recommend the use of skin-patch testing in high-risk cases.

Aged↗
PubMed1995

Prediction of clinical outcome of THR from migration measurements on standard radiographs. A study of cemented Charnley and Stanmore femoral stems.

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.

Arthritis, Rheumatoid↗
PubMed1995

An audit of orthopaedic training.

The changing face of clinical practice in general, and of junior doctors' expectations in particular, will inevitably alter the length and nature of higher training. Greater awareness and tighter control of this will therefore be necessary. This analysis of the curricula vitae of 50 orthopaedic surgeons-in-training who were appointed to senior registrar posts at The Royal National Orthopaedic Hospital, has allowed identification of the 'normal' pattern of educational achievement against which individuals may now be assessed. This has potential wide-ranging educational and financial implications.

Adult↗
PubMed1995

Failure of the Ring polyethylene uncemented acetabular cup.

We report a high rate of failure of the Ring polyethylene cementless cup caused largely by granulomatous osteolysis. We have reviewed 126 prostheses inserted from 1986 to 1992 at from 11 to 90 months after surgery. There was radiological evidence of osteolytic granulomas adjacent to the external surface of the cup in 32%, appearing on average at three years from operation. In a subgroup of 59 prostheses followed for at least four years the incidence of such changes was 54%. A total of 27 cups (22%) have required revision, 21 for granulomatous loosening at an average follow-up of five years. In the retrieved prostheses there was obvious polyethylene abrasion and histological examination confirmed the presence of polyethylene wear debris. We found no significant correlation of osteolysis with cup size, although smaller cups were predominant among those having revision.

Acetabulum↗
PubMed1995

Danger to the popliteal artery in high tibial osteotomy.

We report a case in which the popliteal artery was divided during upper tibial osteotomy performed with the knee in 90 degrees of flexion. This position is believed to allow it to fall safely back from the tibia, but we could find no published confirmation. We used duplex ultrasonography in ten healthy volunteers to measure the distance from the popliteal artery to the posterior surface of the tibia at various degrees of flexion of the knee. Our results showed that in 12 of 20 knees the popliteal artery was closer to the tibia in 90 degrees of knee flexion than in full extension. Surgeons performing upper tibial osteotomy should be aware that flexing the knee does not protect the popliteal artery from injury.

Aged↗
PubMed1994

Laxity and function in knee replacements. A comparative study of three prosthetic designs.

This study was designed to determine the influence of laxity on clinical function after knee replacement. Using a recently developed computerized method, anteroposterior laxities were measured in 29 Insall-Burstein posterior stabilized, 25 Kinemax Condylar (posterior cruciate retaining), and 10 Oxford bicompartmental (anterior and posterior cruciate retaining) knee replacements. Laxities of less than 5 min, irrespective of implant design, was associated with an impaired range of passive motion and an increased likelihood of incurring a flexion deformity in excess of 4 degrees. Clinical function, as reflected by the American Knee Society Clinical Rating system, demonstrated a particular sensitivity in the Oxford knee to anteroposterior laxity. It was concluded that anteroposterior laxity in excess of 5 mm in prosthetic knees is desirable for unimpaired joint function, although an upper limit of acceptable anteroposterior laxity could not be identified. These findings, which emphasize the need for attention to ligamentous tensions at the time of surgery, also indicate the need for appropriate weighting of anteroposterior laxity in the continuing evolution of clinical rating systems.

Adult↗
PubMed1994

Knee function after patellectomy. A 12- to 48-year follow-up.

We describe 83 knees (69 patients) which had had patellectomy for anterior knee pain (52), patellofemoral osteoarthritis (25) or comminuted fractures (6) between 1942 and 1978. The patients were questioned about their symptoms and the function of the operated knee 14 to 50 years after operation. In the group with anterior knee pain, 76% achieved good results and were satisfied with the operation. Only 54% of the osteoarthritis group had satisfactory relief of pain and most had progressive deterioration of function. Sixteen patients who had had unilateral patellectomy were assessed by dynamometry, ultrasound and radiography. The average quadriceps muscle power was 60% of that on the normal side although two patients had stronger muscles in their operated than in their unoperated legs.

Adult↗
PubMed1993

Proprioception after knee arthroplasty. The influence of prosthetic design.

It is recognized that awareness of joint position in the knee deteriorates because of aging, anterior cruciate injury, or osteoarthrosis. Proprioception was studied in osteoarthritic and prosthetic knees, including both posterior cruciate ligament (PCL)-retaining and PCL-sacrificing designs. An improvement in proprioception was observed after knee arthroplasty. A PCL-retaining prosthesis was found to confer a greater improvement than a PCL-sacrificing design.

Adult↗
PubMed1991

Joint proprioception in normal, osteoarthritic and replaced knees.

We measured joint position sense in the knee by a new method which tests the proprioceptive contribution of the joint capsule and ligaments. The leg was supported on a splint, and held in several positions of flexion. The subjects' perception of the position was recorded on a visual analogue model and compared with the actual angle of flexion. Eighty-one normal and 45 osteoarthritic knees were examined, as were 10 knees with semi-constrained and 11 with hinged joint replacements. All were assessed with and without an elastic bandage around the knee. There was a steady decline in joint position sense with age in subjects with normal knees. Those with osteoarthritic knees had impaired joint position sense at all ages (p less than 0.001). Knee replacement improved the joint position sense slightly (p less than 0.02); semi-constrained replacement had a greater effect than hinged replacement. The effect of an elastic bandage in subjects with poor position sense was dramatic, improving accuracy by 40% (p less than 0.001). It is proposed that reduced proprioception in elderly and osteoarthritic subjects may be responsible for initiation or advancement of degeneration of the knee.

Adolescent↗
PubMed1991

Unicompartmental knee arthroplasty. Eight- to 12-year follow-up evaluation with survivorship analysis.

One hundred consecutive unicondylar knee arthroplasties were reviewed after eight to 12 years of follow-up evaluation. Eighteen patients (19 knees) had died, four patients (four knees) were lost to follow-up evaluation, and 13 patients (13 knees) had revision. Survivorship analysis revealed 90% survivorship of the prostheses at nine years, 85% at ten years, and 82% at 11 years. Sixty-four knees in 51 patients were studied clinically and roentgenographically at final follow-up study. Of these, 87% had no significant pain. The average knee flexion was 115 degrees. Anatomic knee alignment averaged 3 degrees of valgus for the knees with preoperative varus alignment and 8 degrees of valgus for knees with preoperative valgus alignment. Fifteen percent of these elderly patients (mean age, 80 years) used a cane outdoors, but only 8% because of their knee. Sixty percent had radiolucent lines at the tibial bone-cement interface, and these lines were incomplete in 96% of cases.

Aged↗
PubMed1986

Screw fixation of subcapital fractures of the femur--a better method of treatment?

A series of 71 patients with subcapital fractures of the femur treated by internal fixation with three AO cancellous screws is reviewed. Sixty-five patients survived and were followed up for 26-73 months from injury (mean 47 months). The incidence of non-union was 4.7 per cent. The incidence of avascular necrosis was 33.3 per cent in displaced fractures and 7.7 per cent in undisplaced fractures. Early weight bearing did not affect the incidence of non-union, avascular necrosis or regression of the screw. We recommend internal fixation of subcapital fractures of the femur by this method.

Adult↗
PubMed1975

The effect of in utero decapitation on the morphological and physiological development of the fetal rabbit lung.

A study has been made of the consequences of in utero decapitation on the morphological and physiological development of the fetal lung. Fetal rabbits were decapitated in situ at 22 days, without losing any amniotic fluid, and allowed to continue their development with their undamaged littermates as controls. Such decapitation, of course, removes the pituitary and so interferes with adrenal cortical development. Morphological studies showed an interference with lung development in that, although the number of alveolar saccules increased normally, their walls failed to thin. In the decapitated fetuses, a reduction in the number of lamellated bodies per Type II pneumonocyte was found at each age studied; while dense, homogeneous bodies were more numerous. The normal disappearance of glycogen in the Type II pneumonocytes of the decapitated fetuses was retarded. Physiological studies supported these findings. In control fetuses allowed to breathe for a while the Bubble Stability Ratio increased rapidly from day 26 to reach a maximum at 28 days; whereas, in the decapitated ones, bubble stability was not apparent before day 28 and by the 29th day had reached a maximum which was lower than that of the controls. In the control fetuses, lecithin was detected in lung fluid from 26 days on, and in stomach fluid from 29 days. It is argued that lung development must be, at least in part, under the control of the fetus' own pituitary-adrenal axis.

Animals↗