A combined intra- and extra-articular reconstruction using a carbon-dacron composite prosthesis for chronic anterior cruciate instability

International Orthopaedics (SICOT) (1991) 15:219-227 International Orthopaedics

© Springer Verlag 1991

A combined intra- and extra-articular reconstruction using a carbon-dacron composite prosthesis for chronic anterior cruciate instability

A two to six-year follow-up study

P. M. Aichroth j, D. V. Patel l, C. B. Jones 2, and J. S. Wand 3

1 Department of Orthopaedics, Westminster Hospital, Horseferry Road, London SW1P 2AP, United Kingdom 2 Royal United Hospital, Combe Park, Bath BA1 3NG, United Kingdom 3 Queen Mary's Hospital, Roehampton, London SWl5, United Kingdom

Summary. Fifty patients with chronic symptomatic anterior cruciate insufficiency underwent liga- mentous reconstruction using the "Westminster Composite Prosthesis" which consists o f a central core of carbon fibre with an external weave o f polyester. In 42patients, the prosthesis was used as an augmentation within a tube o f ilio-tibial tract, combined with a Macintosh's extraarticular reconstruction. The prosthetic replacement alone was used in eight patients where there had been a previous extraarticular reconstruction and was associated with only fair results. The average age o f the patients at operation was 28.5years (range 18 to 50years); instability o f the knee had been present

for a mean of 5.3years and the average follow-up was 3.8years (range 2 to 6years). Assessment in- cluded subjective functional rating (Lysholm knee score) and clinical examination for instability. Thirty-seven patients (74%) had a good or excellent result, 11 (22%) had a fair result and two (4%) had a poor result. The Lachman test was grade I or less in 35patients and the pivot shift sign was eliminated in 37patients. Clinical signs of instability correlated well with the Lysholm knee score (p < 0.001). Twenty-two unselected patients (44%) underwent an arthroscopic assessment and biopsy o f the "neoligament" at an average o f 10.4 months post-operatively. The prosthesis was found to be stable and well covered by a thick fibrous sheath ("neoligament") in 19patients. The prosthesis was partially ruptured in two patients and completely disrupted in one. Thirty-two patients (64%) returned to their previous sports and 13 of them (26%)

Reprint request to: P. M. Aichroth

achieved their pre-injury level o f performance. Overall, 45patients (90%) were pleased or satisfied with the results o f their knee reconstruction.

R6sum6. Cinquante malades souffrant d'une instabilitb antOrieure chronique du genou ont bbndficib d'une reconstruction ligamentaire ~ l'aide d'une ~(prothOse composite de Westminster~ comportant un axe en fibres de carbone entourd d'une gatne de polyester. Chez 42 patients la prothOse a btb utilisOe comme complbment d'une greffe de fascia lata, as- socibe g~ une reconstruction extra-articulaire selon la technique de Macintosh. La prothOse n'a dtd em- ploybe isoldment que dans huit cas, qui avaient db- jgz subi une intervention extra-articulaire avec un rb- sultat seulement passable. L'@e moyen des malades au moment de l'opbration dtait de 28 ans et demi (de 18 gl 50); l'instabilitb durait depuis 5.3 ans en moyenne et le recul moyen btait de 3.8 ans (de 2 gl 6 ans). Le bilan a comportb une estimation sub-

jective de la fonction (selon la cotation du genou de Lysholm) et une estimation clinique de la stabilit& Le rOsultat btait excellent ou bon chez 37 opbrbs (74%), moyen chez 11 (22%) et mauvais chez 2 (4%). Le signe de Lachman btait cotb gl 1 ou moins chez 35 malades et on ne retrouvait pas d'instabilitb ro- tatoire chez 37 malades. Les signes cliniques d'instabilitb btaient bien correlOs avecla cotation du genou de Lysholm. Vingt-deux patients non sblec- tionnOs ont subi un examen arthroscopique avec biopsie du (¢nbo-ligament~ 10.4 mois en moyenne aprOs l'opOration. La prothdse a Otd trouvbe stable et bien recouverte d'une @aisse eouche de tissu fi- breux ((~nOo-ligament~) chez 19 d'entre eux. La prothose Otait rompue partiellement chez deux malades et complbtement chez un troisiOme. Trente-deux patients (64%) avaient repris leur aetivitO sportive an-

220 P. M. Aichroth et al.: Composite prosthesis for chronic ACL instability

FOOTBALL

RUGBY 9

orMil~a

8

Fig. 1. Mode of injury

SQUASH ,3.

HER 9

tdrieure et 13 d'entre eux (26%) avaient retrouv6 le niveau qu'ile attoignaiont avant l'aceident. Dans l'ensemble, 45 sujets (90%) btaient satisfaits du rd- sultat de l'intervention.

lO0-

g0-

80-

70-

"~ 60 -

50- "6 40-

30-

20-

10-

O"

r--" l Pre-operative 92% Post-operative

7O%

Grade I Grade 2 Grade 3 or less

Lachman test

r ~ l Pre-operative Post-operative

7407° 66%

34o70

Grade Grade 1 Grade 2 Grade 3

Pivot shift sign

Fig. 2. A histogram showing the pre- and post-operative results of the Lachman and pivot shift test

greater stability to allow a full active life or return to sport.

We have undertaken a retrospective study of 50 patients with chronic ACL deficiency to deter- mine the efficacy of a combined intra- and extra- articular reconstruction.

Material and methods

Introduction

Rupture of the anterior cruciate ligament (ACL) has been called the "beginning of the end for the knee" [8]. Significant numbers of patients with chronic ACL deficiency complain of recurrent giving way, pain and effusion which limit sporting activities and eventually interfere with the activities of everyday life. Many patients with such instability make fewer demands upon their knees and merely require rehabilitation. A few demand

Table 1. Previous or concurrent operations

Procedures Previous Concurrent

Arthroscopic partial medial meniscectomy 12 0

Arthroscopic partial lateral meniscectomy 6 0

Open medial meniscectomy 6 0

Open lateral meniscectomy 3 0

Arthroscopic trimming of medial meniscus 0 7

Arthroscopic trimming of lateral meniscus 0 5

Macintosh procedure 8 42

Medial collateral ligament repair 2 0

Pes Anserinus transfer 2 0

Carbon fibre reconstruction 2 0

Between 1982 and 1986, 53 consecutive operations for chronic ACL deficiency were performed or supervised by the senior author. Three patients were lost to follow-up, leaving 50 avail- able for the review. All patients were examined personally by D. V.P. Fifty knees (27 right and 23 left) in 50patients (41 males and 9 females) were seen at an average of 3.8 years (range 2 to 6 years) after operation. Their mean age at operation was 28.5 years (range 18 to 50 years). The average dura- tion between the initial injury and the reconstruction was 5.3 years (range 1 to 16 years).

Mode and mechanism of injury

Soccer and rugby accounted for 60% of the injuries (Fig. 1). All 50 patients had participated in some sport before operation but only 33 played contact games. The diagnosis of ACL rupture was rarely made at the initial presentation [24, 27]. One or more further significant injuries occurred in 24% of the patients within two years of their initial damage.

Indications for operation

The principal indication was progressive symptomatic instability in an active, well motivated young individual, who failed to improve with physiotherapy, bracing or arthroscopic correction of meniscal lesions. Patients who would not mod- ify their sporting life style were also considered.

Previous surgery Twenty-eight patients had undergone previous operations on their knees (Table 1). Two had failed carbon fibre reconstructions.

P. M. Aichroth et al.: Composite prosthesis for chronic ACL instability 221

Table 2. The Lysholm scoring system (Lysholm and Gillquist 1982)

Table 3. The scoring system for clinical examination at review: (maximum points = 50)

Points Points allocated

Limp None 5 Slight or periodical 3 Severe and constant 0

Support None 5 Stick or crutch needed 3 Weight bearing impossible 0

Stair climbing No problem 10 Slightly impaired 6 One step at a time 2 Unable 0

Squatting No problem 5 Slightly impaired 4 Not beyond 90 ° of flexion 2 Unable 0

Walking, running and jumping

A. Instability Never giving way 30 Rarely during athletic or other severe exertion 25 Frequently during athletic or other severe exertion 20 Occasionally in daily activities 10 Often in daily activities 5 With every step 0

B. Pain None 30 Inconstant and slight during severe exertion 25 Marked on giving way 20 Marked during severe exertion 15 Marked on or after walking more than two kilometres 10 Marked on or after walking less than two kilometres 5 Constant and severe 0

C. Swelling None 10 With giving way 7 On severe exertion 5 On ordinary exertion 2 Constant 0

Atrophy of thigh None 5 One to two centimetres 3 More than two centimetres 0

Lachman test

Grade 0 15

Grade 1 10

Grade 2 5

Grade 3 0

Pivot shift test

Grade 0 30

Grade 1 20

Grade 2 10

Grade 3 0

Range of movement

Within 5 ° of other knee 5

5°-10 ° less than other knee 3

> 10 ° less than other knee 0

medial collateral ligament laxity and two had an associated posterior cruciate ligament deficiency.

Post-operative assessment

(1) Functional and clinical assessment

The Lysholm scoring system [21] was used to as- sess symptoms (Table 2). The clinical assessment was graded as shown in Table 3. A combined score, with a maximum of 100 points, was calcu- lated for each patient. This was derived from the sum of the Lysholm score (100 points) and the clinical score (50 points) multiplied by 2/3.

(2) Arthroscopic and histopathological assessment

Twenty-two patients underwent arthroscopic examination at an average of 10.4 months after the reconstruction to evaluate the stability of the "neoligament", the degree of fibrous ingrowth within the prosthetic ligament, the synovium, the menisci and the articular surfaces. A biopsy of the "neoligament" was taken.

Pre-operative assessment

All patients had pain, recurrent swelling, giving way and limitation of function in the activities of daily living or sport. Each had an examination under anaesthetic and arthroscopy prior to the reconstruction. The Lachman test [31], the anterior drawer sign and the pivot shift sign [13] were graded on a scale of zero to three, zero being regarded as normal (Fig. 2). Eight patients had mild

(3) Cruciometer assessment

Quantitative evaluation of the Lachman test was made using the "WeStminster Cruciometer ' , which measures the anterior subluxation of the tibia on the femur when an 89 Newton force is applied 10 cm distal to the joint-line, countered by an equal force to the patella (Figs. 3 and 4).

The "Westminster Composite Ligament". The "Westminster Composite Prosthesis" consists of a


Fig. 3. The "Westminster Cruciometer" has been applied to the leg

Fig. 4. An 89 Newton force was applied to the tibia, 10 cms distal to the joint-line. The forward subluxation was recorded on the vernier gauge

s leeve of spec ia l ly t e x t u r e d a n d t r ea ted po lyes t e r w h i c h p r o v i d e s a n o p e n n e t w o r k of f ibres for t i ssue i ng rowth , whi le a f f o r d i n g c o n s i d e r a b l e p ro - t e c t i o n for the c a r b o n f ibre core. The c a r b o n f ibre is a pa ra l l e l a s s e m b l y o f 40,000 p u r e h igh s t r eng th f i l a m e n t s ( A p p e n d i x 1). The pros thes i s has a flex- ib le p las t i c p r o b e a n d a po lyes t e r t ape l eade r l o o p to a id i m p l a n t a t i o n .

Operative technique The patient was placed in a supine position with the affected knee flexed to about 90 °, over the end of the operating table, with a sand bag placed under the thigh. A high tourniquet was applied following exsanguination. A skin incision was centred

over the lateral aspect of the thigh, commencing proximally from a point 10 cm above the supracondylar region, extending distally along the femoral shaft, through the midcondylar point, and terminating at Gerdy's Tubercle (Fig. 5). A strip of ilio-tibial tract 2 cm wide and 16-20 cm in length was raised and detached proximally, remaining attached to Gerdy's tubercle distally (Fig. 6). A Macintosh extra-articular tenodesis was performed with the ilio-tibial tract passed beneath the fibular collateral ligament, after isolating the latter by extra-synovial dissection. With the knee in flexion and the tibia in 30 ° of external rotation, the ilio-tibial tract was kept taut and four Vicryl sutures positioned, but not tied at this stage (Fig. 7).

A drill hole was made in the femoral supracondylar mid- point, emerging posteriorly just above the capsule (Fig. 8). The prosthesis was passed through this bony tunnel and a toggle inserted in the loop. The ilio-tibial tract was tubed over a "T" shaped cannula, using a fine absorbable suture with a continu- ous, interlocking stitch (Fig. 9). The leading loop of the prosthesis was then drawn through the tubed ilio-tibial tract and an encircling transfixion suture was taken over the end of the tube and the prosthesis.

Using blunt dissection, the space immediately posterior to the lateral head of the gastrocnernius was exposed. A curved director with a nylon tape was passed through the posterior capsule, into the intercondylar region of the knee joint. A medial parapatellar arthrotomy incision was made. The tube of ilio-tibial tract surrounding the prosthesis was passed "over the top" of the lateral femoral condyle, through the intercondylar notch, into the cavity of the knee joint. Using a tibial drill guide, a tunnel was made through the antero-medial tibia to the insertion of the ACL on the tibial plateau (Fig. 10). The proximal end of the tibial tunnel was made smooth using a high speed burr and a curette. The tube of ilio-tibial tract and prosthesis was then passed through the tibial tunnel and a cor- tical bone block (3 cm x 1 cm) raised from the medial aspect of the proximal tibia, just inferior to the exit hole of the tibial tunnel. The prosthesis was then pulled tight with the tibia ex- ternally rotated on the femur and drawn posteriorly. The isometricity of the prosthesis was assessed by flexing and extending the knee. The four stay-sutures anchoring the ilio-tibial tract to the fibular collateral ligament were tied. The tubed prosthesis was placed where the bone block had been raised, and the block stapled into position with the prosthesis under maximal tension (Fig. 11).

Post-operative management

A well padded above knee cast was applied with the knee in 40 ° of flexion and the tibia in external rotation. The patient was mobilised with crutches and the cast was maintained for six weeks. After plaster removal the patient was subjected to a well supervised intensive physiotherapy programme. Between three and six months, fast walking, jogging and swimming were encouraged. At six months, the patient was allowed to participate in mild recreational sporting activities; at nine months, athletic training began and at 12 months all contact sports were permitted.

Results

(1) Functional assessment

Knee pain. At rev iew 27 p a t i e n t s h a d n o pa in , 15 h a d o c c a s i o n a l or s l ight k n e e p a i n d u r i n g severe exe r t i on a n d eight h a d m a r k e d p a i n o n g iv ing


Figs. 5-11. Illustrations demonstrating the operative technique

way or during severe exertion. None had knee pain at rest.

Swelling. Fifteen patients had a minor effusion on severe exertion and two had a persistent effusion.

Instability. Twenty-seven patients stated that their knee did not give way. Sixteen had occasional giving way during athletic or other severe exertion and six had frequent episodes of giving way with similar activities. Only one patient complained of occasional giving way during daily activities.

Limp. Forty-four patients had no limp at the time of review and six had a slight limp.

Stair climbing. One patient had slight difficulty on climbing stairs.

Squatting. Eight patients had slight impairment in squatting.

(2) Clinical assessment

Stability. (Fig. 2) The Lachman test was grade 1 or less in 35 patients and grade 2 in 15 patients. The anterior drawer test was grade 0 in 24 patients, grade 1 in 22 patients and grade 2 in four patients. The pivot shift sign was eliminated in 37 patients. Twelve patients had grade 1 pivot shift glide but no jerk, and one had grade 2 pivot shift with a jerk.

224

Table 4. Complications

Number

Rupture of the implant Partial 2 Complete 1

Superficial wound infection 1 Persistent effusion 3 Flexion contracture 1 Limitation of terminal flexion 5 Extension lag 2 Tenderness around staple 2 Muscle herniae 3 Total 20

Range o f movement. Forty-two patients had a full range of movement (0 to 140°), five had a 10 ° re- striction of terminal flexion and two had an extension lag of 10 °. One patient had a flexion contracture of 10 °.

Thigh atrophy. Quadriceps wasting was less than 1 cm in five cases. Forty-three patients had 1-2 cm of wasting and two had more than 2 cm of wasting, measured at a point 10 cm above the superior border of the patella.

(3) Cruciometer assessment

The mean cruciometer reading for patients who had been assessed as having a Lachman test of grade 1 or less was 4.56 mm (range 3.6-5.6 mm). Patients with a grade 2 Lachman test had a mean excursion of 6.62 mm (range 5.8-7.6 mm). There was a highly significant correlation between the Lachman test and the cruciometer reading (p <0.001, r = 0.76).

(4) Overall score

A score of more than 90 was regarded as excellent, 78-90 as good, 66-77 as fair and less than 66 was considered a poor result.

Sixteen patients had an excellent result, 2! were good, 11 fair and 2 poor. There was a statis- tically significant correlation between the Lys- holm score and the clinical score (p <0.001, r = 0.60). Patients with a combined intra- and extra-articular reconstruction fared significantly better (Student's "t'" test, p < 0.001) than those with an intra-articular reconstruction alone.

P. M. Aichroth et al.: Composite prosthesis for chronic ACL instability

actively participating in contact games. The remaining nine patients had changed their lifestyle and were restricted to the ordinary activities of daily living; four of these were unwilling to ex- pose their symptom-free knees to the risk of further injury. Overall, 32 patients (64%) returned to their previous sporting activities and 13 (26%) achieved their pre-injury level of performance.

Complications

There was one superficial infection which re- solved with antibiotics. Seventeen patients had a total of 20 complications (Table 4). The prosthesis was completely disrupted in one patient and partially ruptured in two.

None of the patients underwent revision of the reconstruction. Two patients had subsequent arthroscopic partial meniscectomy and one had an arthroscopic irrigation and debridement of the knee following partial disruption of the implant.

Overall patient satisfaction

Twenty-five patients (50%) were delighted with the functional result of their knee reconstruction, 20 (40%) were satisfied and five (10%) expressed reservations about the outcome of the procedure.

Arthroscopic and histopathological assessment

Arthroscopy in 22 knees at an average of 10.4 months following reconstruction revealed that the prosthesis was stable and covered by a thick fibrous sheath (Fig. 12) in 19 cases. It was partially ruptured in two patients, 2 and 3 years after operation and had completely disrupted in 1 patient 3 years after reconstruction. These three patients had received a ligament substitution only without a Macintosh tenodesis. There were signs of superficial vascularity in all the intact "neoliga- ments".

Arthroscopic biopsy from the fibrous en- velope over the tubed composite prosthesis showed a low grade synovitis with a mild foreign body giant cell response. One section taken from a patient with a complete rupture of the prosthesis showed a mixed foreign-fibre granuloma with moderate carbon fibre deposits and refractile fragments of dacron in the subsynovial layer ex- citing a mild foreign body giant cell and fibrous response(Fig. 13).

(5) Return to sports

At the last follow-up, 30 patients (60%) were still involved in recreational sports and 11 (22%) were

Associated pathology

The incidence of lesions of the meniscus and articular cartilage found at arthroscopy before re-


Fig. 12. Arthroscopic appearance at six months. The hook is probing the mid-point of the "neoligament".

construction, at the time of operation and after reconstruction is shown in Table 5.

Discussion

There is debate concerning the natural history of ACL rupture and the need for surgical reconstruction [10, 22, 29]. Many surgeons feel that with intensive rehabilitation the majority of patients can

return to a high level of sporting activities. How- ever, marked rotatory instability and a pivot shift jerk make sports requiring control in twisting and cutting impossible. Functional deterioration oc- curs with time [24], and if a patient with marked ACL deficiency continues with an active sports career, progressive damage to the menisci and the articular surfaces will result in a slow onset of os- teoarthritis. Knee stabilization is desirable.

Many procedures have been described for the reconstruction of the torn ACL. There has been continued debate over the efficacy of extra-articular procedures rather than intra-articular reconstruction. The Macintosh extra-articular tenodesis has shown good short-term stability [5] and en- couraging long-term results [2, 11]. However, Elli- son [9] reported 55% excellent initial results using a distal ilio-tibial band transfer, falling off to 44% at two years. Kennedy et al. [20] and Andrews [4] both reported a substantial number of their patients with a continued pivot shift jerk following an extra-articular reconstruction in high performance athletes.

The long-term success of autogenous grafts is unpredictable. Reconstructive procedures utilis- ing autogenous tissues often require a prolonged rehabilitation and may have a failure rate of 30-40% at five years [12, 26]. Utilization of autol- ogous ligamentum patellae may weaken the donor site and lead to functional disability, especially in athletes [6]. Nevertheless, this procedure is popu-

Fig. 13. Histology of "neoligament" from a patient with a ruptured prosthesis. Foreign body giant cell reaction to anisotropic fibres of Dacron and occasional carbon fibres (Haematoxylin and Eosin, half polarized light × 41)


Table 5. Associated Pathology: The number of meniscal and articular cartilage lesions seen at arthroscopy before, during and after reconstruction

Site of lesion Before At After operation operation operation a

Meniscus Medial 12 7 1 Lateral 6 5 1 Both 2 1 0

Articular cartilage Medial femoral condyle 2 3 1 Lateral femoral condyle 1 2 0 Patella 0 2 1

~, Only 22 of the 50 patients had a post-reconstruction arthroscopy

lar and the pate l lar p r o b l e m s m a y be overs ta ted. K e n n e d y et al. [20] showed that in some instances the pate l lar t endon graf t m a y e longate or fail u n d e r low forces, or r ema in in the jo in t as f l imsy, co l lagenous tissue p rov id ing no l igamentous re- straint. J ohnson et al. [18] reviewed 87pa t i en t s t rea ted by a mod i f i ed Jones p rocedu re at an average of 7.9 years and found that 67.9% had loss o f knee extension, which was f requent ly the cause of an unsa t i s fac tory result. However , m a n y surgeons feel that a bone-pa te l l a r t e n d o n - b o n e reconst ruc- t ion shows consis tent re turn to spor t ing activities.

F i l amentous ca rbon has been studied exten- sively as a scaffolding mater ia l fol lowing the work of Jenkins [17]. Before 1983, ca rbon fibre pros theses were used for the recons t ruc t ion o f chron ic A C L instabi l i ty at Wes tmins te r Hospi ta l . However , Aichroth et al. [1] r epor ted that 76.5% of the implan t s passed th rough a drill hole in the lateral f emora l condyle were b r o k e n at a r th roscopy at a m e a n o f seven mon ths fo l lowing opera t ion , whereas all the c a r b o n - d a c r o n compos i t e pros- theses that were routed "ove r the t op" o f the lateral f emora l condyle and s u p p l e m e n t e d with a M a c i n t o s h tenodesis were intact at a m e a n of 24 months . C a r b o n fibre is brit t le and breaks unde r twisting or angu la r forces; there are specif- ic stresses at the p ros thes i s -bone interface. Rush- ton et al. [28] found that c a rbon fibre had not in- duced the f o r m a t i o n o f a " n e o l i g a m e n t " and the prosthesis was mere ly covered by a thin f ibrous sheath. The c a r b o n - d a c r o n compos i t e prosthesis used in the presen t series showed the f o r m a t i o n o f a thick f ibrous tissue a r o u n d the dac ron ("neol i - gamen t " ) in 19 of the 22 a r th roscop ic examina- tions.

T h o m a s et al. [30] and Amis et al. [3] have d e m o n s t r a t e d that dac ron is b iocompa t ib l e , mechanical ly s t rong and exhibits good pene t ra t ion

with f ibrous connec t ive tissue. We covered the central core o f c a rbon fibre with the two protec- tive layers of dac ron and il io-tibial t ract to pro- vide the knee with secure p ro tec t ion f rom ca rbon fibre staining. Severe p rob l ems m a y deve lop if in t ra-ar t icular ca rbon fibre a lone is employed , no tab ly c a r b o n fibre s taining of the synov ium, ar- t icular cart i lage and the menisci , which can result in chronic synovit is and effusion. The presence o f an ilio-tibial t ract tubed dac ron coat ing elimi- na ted the f r agmen ta t i on and sp read of c a rbon fibre debris within the joint , p rov ided that the " n e o l i g a m e n t " r ema ined intact. The f ibrous ingrowth of the dac ron coat ing m a y p rov ide fur ther mechan ica l r e in fo rcemen t of the prosthesis as well as effect ive c o n t a i n m e n t of the central carbon f ibre core. All three pat ients with a par t ia l or a comple te rup ture o f the prosthesis showed carbon f ibre s ta ining of the in t ra-ar t icular structures.

We p re fe r red the "ove r the t op" route as it lo- cated the prosthesis close to the ana tomica l origin o f the A C L [1, 23, 32] and a l lowed the imp lan t to pass over a round , smoo th surface. The opera t ive technique descr ibed was s imple to p e r f o r m and has p r o d u c e d consis tent results.

Appendix 1.

Mechanical data on the "Westminster Composite Prosthesis"

The mechanical strength of the prosthesis relies on the central core of the carbon fibre, and not the outer covering of the open weave of Dacron. Accordingly, only the carbon fibre component of the prosthesis was tested mechanically.

Number of filaments per tow 40,000 Filament diameter 6.8 × l0 -6 m. Tensile strength, 0.168 N/decitex. Tow strength" 4480 N.

a Both these tests were conducted at a relative humidity of 60%, at 16°C

Acknowledgements. The authors would like to thank Dr. A. C. Branfoot for his help in the interpretation of the histology. Our thanks are also due to Dina Stanford and the Medical Photo- graphy Department at Westminster Hospital for their help with the illustrations and to Linda Terrett for secretarial assistance. We are grateful for the financial assistance from the Orthopaedic Department Research Fund of Westminster Hospital.

No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.

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Documents

A combined intra- and extra-articular reconstruction using a carbon-dacron composite prosthesis for chronic anterior cruciate instability