Graft fixation issues in kneeligament surgery

GRAFT FIXATION ISSUES IN KNEE LIGAMENT SURGERY JEFF BRAND, JR, MD, ANDREAS a n d D A R R E N L. J O H N S O N , M D

WEILER,

MD, DAVID N.M. CABORN,

MD,

The popularity of soft tissue grafts, particularly the semitendinosus and gracilis, has raised several issues with regard to graft fixation in cruciate ligament reconstruction. What is the force required by activities of daily living and a progressive rehabilitation program? Does soft tissue fixation alter the biological process of graft incorporation compared with the use of a bone plug? This article reviews the mechanical properties and use of fixation devices. Interference screw fixation of a patella tendon bone plug has been clinically reliable. Relative ease of fixation, acceptable initial strength, and fixation close to joint surfaces contribute to the popularity of cruciate ligament reconstruction with patella tendon bone plug. Biodegradable interference screw fixation of a bone plug is a reasonable alternative to metal screw fixation, which has several disadvantages. Many forms of tendon fixation are placed at a distance from the joint and rely on linkage materials, resulting in graft tunnel shear forces and possible tunnel expansion. Direct interference screw fixation may alleviate these detractions, but ultimate failure at time zero, particularly tibial fixation, may not allow for a progressive rehabilitation program, which our patients currently benefit from with interference fixation of a bone plug. Diminishing individual fiber movement within the tendon graft and the elimination of linkage materials will improve future soft tissue fixation. KEY WORDS: cruciate ligament, graft fixation, interference screw

Although anterior cruciate ligament (ACL) reconstruction has produced good subjective results, and return to activities has been gratifying, some patients have experienced residual laxity. TM This problem may result from initially weak graft fixation that occurs with current methods. No commonly used graft fixation has ultimate failure strength or stiffness comparable with the native cruciate ligament. 5,6 There is a lack of comparative clinical series of alternative graft fixation choices with similar rehabilitative programs. The majority of present research consists of testing various constructs at time zero. The rate of application of force, direction of applied force, presence of preload, use of animal and elderly cadaver models, and absence of graft conditioning vary among studies. The direct applicability of these pull-out studies to the clinical outcome of cruciate ligament surgery remains unproven. The autologous patellar tendon has significant donor site morbidity. An allograft patellar tendon is an increasingly scarce commodity because of the popularity of this graft choice. Both autograft and allograft soft tissue fixation will be increasingly demanded. Soft tissue fixation, particularly on the tibia, offers room for improvement. Desirable graft fixation is at the anatomic insertion sites rather than deeper in the tunnel or extracortical. 7

From AlexandriaOrthopaedicsand Sports Medicine,Alexandria, MN; Humboldt-University,Berlin,Germany;and Universityof KentuckySchool of Medicine,Lexington,KY. Address reprint requeststo Darren L. Johnson, MD, Section of Sports Medicine, 740 S. LimestoneStreet, K401 KentuckyClinic, Lexington,KY 40536-0284. Copyright© 1999 by W.B. SaundersCompany 1048-6666/99/0904-0002510.00/0

256

IDEAL

GRAFT

FIXATION

Strength

Noyes et al 8,9 estimated, but did not measure, the strength required for activities of daily living (ADLs) to be 454 N. Markolf et aU ° in biomechanical testing, showed that the patella tendon graft experienced higher forces than the native ACL (peak force, 297 N) and overtensioning the graft increased the forces (up to 497 N). The forces in a similar posterior cruciate ligament (PCL) study by the same group were much lower, generally less than 100 N. 11 By using force plate and gait analysis data, Morrison 12-14 made calculations and conclusions regarding the forces in the ACL and PCL (Table 1). There is evidence that less than 454 N is sufficient. In a clinical study, Shelbourne and Gray is used a button for both tibial and femoral fixation of a patella tendon reconstruction, with an ultimate failure strength of 248 N and an accelerated rehabilitation program that produced excellent clinical and objective stability. Biomechanical Properties

Many tendon fixation devices are "indirect"; they rely on linkage material--suture or tape--to connect the tendon to the fixation device. In a biomechanical study that compared creep induced by cyclic loading in a patella tendon graft and a quadrupled hamstring, the tape-tissue interface of the quadrupled hamstring graft (5.4%) had notably more creep than the tape (2.9%) or the tissue (1.1%) alone. 16 Another biomechanical study found 3 to 5 mm of quadrupled hamstring graft tunnel motion with the EndoButton (Smith and Nephew Endoscopy, Andover, MA) linked

Operative Techniques in Orthopaedics, Vol 9, No 4 (October), 1999: pp 256-263

TABLE 1. Estimations of Forces Present in the Cruciate Ligaments in Activities of Daily Living Activities

ACL (N)

PCL (N)

169 67 445 93 27

352 641 262 449 1215

Level walking Ascending stairs Descending stairs Descending ramp Ascending ramp Data from Morrison. ~2-14

with tape, supporting the "bungee cord effect. "17 Furthermore, many tendon fixation constructs are placed at a distance from the joint surface, such as a staple or a screw with a soft tissue washer. It has been hypothesized that these factors may contribute to high shear forces acting at the tunnel wall, which may delay an osseous graft incorporation and may lead to tunnel enlargement or the "windshield wiper" effectJ 8 Increased knee stability at a variety of flexion angles was found with interference fixation placed closer to the joint surface with a bone plug interference fixation, 19 and fixation closer to anatomic attachment sites improved isometry. 2°

Biological Properties It has been stated that bone plug incorporation occurs before tendon in a bone tunnel, al but this assertion has not been proven scientifically. Clancy et al 2a used a bone plug patella tendon bone plug in a rhesus monkey, which was histologically incorporated at 8 weeks after the procedure, but biomechanical testing was not performed until 3 months after the procedure, at which time bone plug was incorporated in the bone tunnel. 22 Time to incorporation has varied from 12 weeks in a dog extra-articular model 23 to 3 weeks in a rabbit intra-articular model for a soft tissue graft. 24 lit a sheep model, it was 12 weeks before the pull-out failure reached 313 N with soft tissue grafts and biodegradable interference fixation, as

Femoral and Tibial Fixation The bone quality and geometry of the tibia differ from that of the femur. 26 The dual photon absorptimetry of the tibial metaphysis is less than the femoral metaphysis in the same knee of elderly cadavers 27 and in young females.28 The line of applied force to the ligament graft is parallel to the axis of the tibial tunnel. The line of force on the graft

is obliquely orientated to the femoral tunnel in the weightbearing position, which is extension and does not become colinear until 100 ° of knee flexion. 29

GRAFT FIXATION ALTERNATIVES Bone

Plug-Tibial

Fixation

Staples. Graft tunnel length mismatch is the primary indication for staple fixation in bone plug fixation. A set of doubled staples in a shallow trough produced comparable failure strength (588 N) and were significantly stiffer (86.3 N) than interference fixation (506 to 758 N), stiffness (49.2 to 54.9 N / m m ) in a young (mean age, 44 years) human cadaver model. However, the incidence of bone block breakage (27%) was significantly greater than with interference fixation. 3° S c r e w s u s e d a s a p o s t . A s c r e w u s e d as a p o s t l i n k e d w i t h suture combined with an interference screw against a bone

plug has a failure strength (674 N) that approximates that of the intact ACL (560 N) in the same study (Table 2).5 The angle at which the screw is placed determines whether the graft is tensioned or relaxed as the screw is tightened. Although a "low-profile" screw is available from many orthopaedic manufacturers, some conventional screws (8%) must be removed because of pain. 6 Interference fixation. Interference fixation

was

first

de-

scribed by Lambert, 31 who used a 6.5-mm cancellous screw. 31 In 1987, Kurosaka et a132showed superior strength with a larger diameter screw used as an interference fixation. The fixation strength required for ADLs and a progressive rehabilitation program appears to be met by the strength and stiffness of interference fixation of a bone plug. 5 In the case of poor bone stock resulting from revision or tunnel widening, graft tunnel length mismatch, and additional fixation strength for large noncompliant patients, interference fixation may be combined with other fixation such as a suture and post, EndoButton, or Mitek device (Westwood, MA). Currently, a screw 9 mm in diameter and at least 20 mm long is recommended. Outside diameter of the screw is more important than core diameter. 2s On the human tibia, Kohn and Rose26 found a significant difference between a

TABLE 2. Tibial Fixation Options for a Bone Plug Construct

Failure (N)

Stiffness (N/mm)

Mechanism of Failure

Patella tendon with suture tied over a button (#5)32 Stapled patella tendon32 Doubled staples on patella tendon in a trough3° Patella tendon with suture and post5 Patella tendon interference screw with 6.5-mm AO screw32 Patella tendon interference with 9-mm "custom" s c r e w 32 Patella tendon with interference screw and suture with a

248.2 _+ 40.2 128.5 + 15.7 588 396 --- 124 214.8 - 39,2 475.8 ± 110.9 674 + 206

12.8 _+ 2.0 10.8 _+ 2.0 86.3 27 _+ 13 23,5 ± 2.9 57.9 ± 3.9 50 + 21

Button failed or suture pulled through the bone plug Graft pulled out from under the staple Graft slipped under staple, 27% bone block breakage Bone-tendon rupture, bone plug fracture, tibial post pull-out Grafts pulled out of the tunnel Grafts pulled out of the tunnel Bone plug fractured, pull-out around tibial screw and suture

461 (230-631) 678 (394-947) 758 + 139 293 (156-458)

47 (28-73) 68 (32-84) 49.2 _+ 2 42 (14-67)

Tendon tearing, slipping of the bone plug Tendon tearing, slipping of the bone plug Tendon tearing or bone plug slippage Bone plug slipped, tendon tearing

rupture

posts

Patella tendon with a 7'-mm interference screw 26 Patella tendon with a 9-mm interference screw26 Patella tendon with a 9 x 30 mm interference screw3° Patella tendon with a 9-mm biodegradable screw27

NOTE: Data from biomechanical studies using human tissue. GRAFT FIXATION IN KNEE LIGAMENT SURGERY

257

9-mm and a 7-mm screw in maximum tensile strength and linear load needed to disengage the fixation screw (Table 2). Until recently, it was thought that screw length greater than 20 m m was unnecessary when used with a bone plug. 33~34 Recent evidence suggests a longer screw may improve fixation. Gerich et al3° noted a statistically significant increase in failure with a 9- × 30-mm screw compared with a 9- × 20-mm screw in the young human cadaver model. 30 The screw can be repositioned against the bone plug with no adverse effect on fixation.3s The shape of the bone plug may increase pull-out failure; a circular wellconforming bone plug has a higher pull-out than a smaller, less conforming trapezoidal bone plug. 36Additionally, the patella fractured at 107% greater force after harvest of a circular bone plug compared with a trapezoidal bone plug. 36 Countertension through the bone plug sutures can reduce graft advancement as the interference screw is placed. 37 If the sutures that are attached to the bone plug are lacerated with the threads from the screw, poor graft fixation cannot be salvaged with a suture and post construct. Graft laceration, although rare, has been reported. 37 Two cases of bone plug comminution have been reported. One was salvaged by reversing the graft and placing the fractured bone plug on the tibial side and fixing it with a suture and post. The other had to be revised with another graft choice.38 Painful hardware was managed by screw removal in 3% of cases in one report, which successfully relieved pain in the area of the tibial screw.39 Biodegradable interference fixation. When a 25-mm long biodegradable screw was used, there was no statistical difference in stiffness (61.8 N / m m v 42.25 N / m m ) or strength (377 N v 293 N) between the femur and the tibia, respectively (Table 2). 40 Two clinical series have compared a metal interference screw to a biodegradable interference screw in a bone-

patellar-tendon graft, one was a retrospective study with a poly (L) lactic acid (PLLA) screw (Arthrex, Naples, FL)41 and the other a randomized, prospective study with another PLLA interference screw (BioScrew; Linvatec, Largo, FL).42 Results were comparable at midterm follow-up evaluation. The BioScrew 7-mm femoral screws fractured at 7%, which did not occur in other groups. The core diameter has since been increased to reduce the possibility of screw breakage on insertion.41

Bone Plug-Femur Press-fit femoral bone plug. Malek et a129press fit the femoral bone plug to avoid the complications of interference screw fixation. Brown et al4a compared the press-fit (350 N) of the bone plug with the patella tendon bone plug with interference fixation, the EndoButton, and the Mitek anchor. No statistical difference was noted in failure or stiffness (Table 3).43 In a clinical study of press-tit femoral fixation, i case of bone plug fracture was encountered, and 2 cases of revision to an interference screw were required because of "insufficient femoral anchorage." The percentage of unacceptable results was 23.3%, which was attributed to anterior placement of the tunnels. 44 Interference fixation. Femoral tunnel blow-out can be salvaged with the EndoButton or the Mitek device. Interfer-

ence screw fixation is considered the "gold standard" for routine femoral bone plug fixation. Two human studies documented similar strength and stiffness between a metal 7-mm diameter screw placed intra-articularly and endoscopically in ACL reconstruction and an outside-in technique with a 9-mm screw (Table 3). 5`33 Although screw divergence from the bone plug is commonly found on postoperative radiographs, 45 it is likely not a significant clinical concern. Dworsky et a146 showed that the endoscopically placed interference screw acts as a wedge, effectively blocking the femoral bone plug from

TABLE 3. Femoral Fixation Options for a Bone Plug Construct

Failure (N)

Stiffness (N/mm)

Mechanism of Failure

Patella tendon fixed with an EndoButton 43

554 _+ 276

27.0 _+ 13.5

Patella tendon, MitekAnchor 43

511 _+ 350

18.3 _+ 8.27

Patellar tendon that is press fit on the femur 43

350 _+ 48

36.8 -+ 16.3

Patella tendon with 9 mm interference screw from out-

423 -+ 175

46 _+ 24

588 -+ 282

3 3 _+ 14

235 +_ 124

82.8 _+ 30.1

Bone plug fractured, pull-out around femoral screw, bone tendon rupture Bone block pull-out, bone block fracture

256 _+ 130

70.2 _+ 28.9

Bone block pull-out, bone block fracture

side-in 6 Patella tendon with 7-mm interference screw placed endoscopically6 Patella tendon with 9-mm interference screw from outside-in 33 Patella tendon with 7-mm interference screw placed endoscopically33 Patella tendon fixed with metal interference screw 52

Fracture of tibial bone block, sutures broke on the tibial side, tibial side fixation failure Patellar tendon failure, fracture tibial bone block, sutures tore through bone block Tibial bone plug pulled out, fracture tibial bone block, patellar tendon failed Pull-out around the screw

558.3 -+ 67.9 No stiffness reported Femoral fixation failure, fracture of bone plug, tearing of

graft Patella tendon fixed with BioScrew interference screw 52 552.5 + 56.4 No stiffness reported Femoral fixation failure, fracture of bone plug, tearing of graft Patella tendon fixed with a metal interference screw 54 640 N -+ 201 No stiffness reported Pull-out and bone block fracture Patella tendon fixed with BioScrew interference screw 54 418 N --- 118 No stiffness reported Bone block pull-out Patella tendon fixed with a metal interference screw 53 436 (111-903) No stiffness reported Failur ebetween the cortical and cancellous bone of the graft Patella tendon fixed with a biodegradable interference 565 (248-987) No stiffness reported Failure between the cortical and cancellous bone of the screw 53 graft NOTE: Data from biomechanical studies using human tissue.

258

BRAND ET AL

being displaced into the joint. Furthermore, an angle of screw divergence from the femoral bone plug greater than 20 ° significantly diminished the pull-out strength in the laboratory. 47 ]In the clinical situation, Fanelli et a148 showed no increase in fixation failure when divergent it~derference screws are placed endoscopically at angles greater than 20 °. Biodegradable d u c e s higher

interference

fixation. Porcine

bone

pro-

failure strengths than human specimens when a metal screw is used for interference fixation. 49 In both a bovine and porcine model, there has not been a significant difference in ultimate failure between a metal interference screw and a biodegradable screw. 5°,51 Two similar studies compared a metal interference screw for femoral interference fixation with a biodegradable interference screw; neither study found a statistical difference between the screws. The biodegradable screws were made by different manufacturers, the BioScrew (Linvatec) 52 and Biologically Quiet from Instrument Makar (Okemos, MI) 53 (Table 3). A third study comparing BioScrew fixation of a bone plug to metal interference fixation has been reported. The metal screw provided superior fixation (Table 3).54 Soft Tissue Fixation-Tibia

and can be recessed to diminish the prominence of the screw head. 49 Biomechanically, this is the only final soft tissue fixation that approximates the ACL in failure strength and stiffness. Metal

interference

fixation. Although

stiffness

may

be

comparable with conventional means of hamstring fixation, a low level of failure and a high level of variability have plagued the biomechanical results of tibial metal interference screw against a quadrupled hamstring graft. 25,49,55-57Screws placed eccentrically or centrally against the graft did not significantly alter the failure or slippage in a composite model. 55In a porcine model, a bone-hamstringbone (BHB) graft was compared with a bone-patellar tendon-bone (BPTB) graft with interference fixation. The BHB failed at a lower level and the amount of graft slippage was greater than the BPTB. s8 Another biomechanical evaluation of a porcine model compared a tendon interference metal screw with a BTB graft fixed with a metal screw; in this case, both constructs were cycled 5,000 cycles at 0.2 Hz. The tendon construct had lower maximal and minimal loads for the same displacement with cycling and had a lower stiffness and lower ultimate failure with static testing after completing cyclic loading. 57 Biodegradable interference fixation. Recent biomechanical

Staples. A single staple used with the semitendinosus is neither strong nor stiff (Table 4). 27 With the "belt-buckle" te&mique, the tendon graft can be looped over a second staple, markedly improving fixation in a porcine model; the failure load was 705 N with a stiffness of 174 N / m n ' t . 49 Frequently, staples cause pain and must be removed. Screws used as a post. A screw can be used with a standard metal washer as a post to tie the suture around, or it can be used with a soft tigsue washer against a tendon. A screw with a spiked soft tissue washer placed directly against a quadrupled tendon graft is slightly stronger and stiffer than the screw used as a post with suture (Table 4).5 The washerplate (WasherLoc; Arthrotek, Biomet, Inc; Warsaw, IN) is a multi-pronged washer and screw used to fix the tibial end of the quadrupled hamstring graft. It is placed at the distal end of the tibial tunnel Washerplate.

studies compared biodegradable and blunt threaded titanium interference screws (RC1; Donjoy-Smith & Nephew, Carlsbad, NM) for use in hamstring tendon interference fit fixation. They found that biodegradable screws provided similar or superior fixation strength to titanium screws. 59-61 However, other studies compared the initial fixation strength of hamstring tendon interference screw fixation with conventional hamstring tendon graft fixation techniques and the interference screw fixation of the BPTB graft. They found substantially lower loads for a hamstring tendon graft fixed with an interference screw. 33,56,62 Recently reported cycling data in a bovine model indicated failure at repetitive loads of less than 150 N for both metal and biodegradable screws. 63 Therefore, some have suggested that the initial strength of transtibial hamstring tendon interference fit fixation may not allow for accelerated postoperative rehabilitation. 49~s6In these reports, the mean failure load of a transtibial ACL reconstruction with hamstring tendons and interference screw fixation exhibited substantially lower loads than the estimated forces in

TABLE 4. Tibial Fixation Options for a Tendon Graft Construct

Failure (N)

Stapled semitendinosus32 137.3 _+ 22.6 Semitendinosus/gracilis quadrupled fixed with suture and post5 573_+109 Semitendinosus/gracilis quadrupled with screw and a soft tissue washer5 821_+219 Semitendinosus/gracilis quadrupled with a washerplate49 905 + 291 Quadrupled ST/G interference screw with the RCI titanium s c r e w 43 214 +_ 78.8 Quadrupled ST/G with interference screw with the RCl titanium screw49 350 + 134 Quadrupled ST/G with interference screw with the RCI titanium screw69 201 + 50.6 Quadrupled ST/G with biodegradable interference screw I mm graft sleeves27 222 _+ 75 Quadrupled ST/G with biodegradable interference screw 1/2 mm graft sleeves27 308 +_ 207

Stiffness (N/mm) 8.8 _+ 1.0 18_+5 29+7 273 _+ 56 8.98 -+ 6.69 248 _+ 52 36.2 No stiffness reported

Mechanism of Failure Tendon pulled out of staple Suture tendon strech, post pull-out Tendon stretchs or tibial screw pulls out No failure mode given Tendons pulled out or slipped No failure mode given Failed at the tibial socket Graft slipped around tibial screw

No stiffness reported Graft slipped around tibial screw

NOTE: Data from biomechanical studies using human tissue. Abbreviations: ST/G, semitendinosus/gracilis. GRAFT FIXATION IN KNEE LIGAMENT SURGERY

259

the native ACL or the graft during daily activities, s,l° However, a clinical report that compared transtibial hamstring with patellar tendon graft interference screw fixation found no significant difference in outcome. 64 Several factors influence the initial fixation strength of hamstring tendon grafts fixed with interference screws. These factors are especially important to increase fixation strength on the tibial site, which has been considered the weak link of such a reconstruction. Initially, Morgan 65 introduced a BHB composite graft for an all-inside ACL reconstruction. However, in a porcine knee biomechanical study, Liu et al 5s found substantially lower loads and a high slippage for this BHB composite graft compared with a BPTB graft, s8 Shin et a166 introduced the harvest of a hamstring tendon graft with a distally attached tibial bone plug, which has been used by Stahelin and Weiler, 67for the tibial fixation of a hamstring tendon graft in an all-inside technique. In a recent biomechanical study, it has been shown that the harvest of a semitendinosus tendon graft with its distally attached bone plug provides fixation strength similar to that provided by conventional BTB graft fixation when both grafts were fixed with biodegradable interference screws. 68 To enhance the direct tendon-to-bone interference fit fixation without bone blocks, a precise match of tunnel size to graft diameter is recommended. A recent biomechanical study compared i and 0.5 mm tunnel sizing and found that sizing the tunnels in increments of 0.5 mm increases fixation strength significantly (Table 4). 61In another biomechanical study that investigated the effect of screw geometry on hamstring tendon interference fit fixation, increasing both screw length and screw diameter significantly improved fixation strength. The influence of screw length (23 mm v 28 mm) was greater than that of thread diameter (screw diameter = graft size v screw diameter = graft size + 1 mm) (unpublished data). Tibial tunnel dilation improved ultimate failure strength of quadrupled hamstring grafts from 323 N to 525 N with direct interference biodegradable interference screw (Arthrex) fixation. 69Fixation of soft tissue grafts correlated with insertion torque of the biodegradable screw in two models--a human quadrupled hamstring fixed with a BioScrew61 and a porcine

tibia with a central quadriceps tendon graft with a biodegradable interference screw (Arthrex). 7° To judge further the appropriateness of this new technique for hamstring tendon graft fixation in cruciate ligament surgery, it is essential to know how tendon-tobone healing progresses under interference screw fixation. Weiler et a171 showed that the healing under interference screw compression follows different patterns from those described in animal models using noncompressing, extraarticular fixation techniques. 23,24In their animal m o d e l the healing did not progress or only partially progressed by development of a so-called fibrous interface, which usually develops between the tendon graft and the bone surface. 71 Rather a direct bone apposition to the tendon graft and the bone surface may exist if compression fixation is used, which may overcome the delayed tendon-to-bone healing if extra-articular fixation is used. When biodegradable interference screw fixation is used for a soft tissue graft, there are concerns about a possible compromise of the graft incorporation when the screw degrades. In the model of Weiler et aL71 an intermediate degrading poly-(D,L-lactide) interference screw was used, which disintegrated macroscopically at 24 weeks. Graft pull-out from the ~ n n e l was not observed, which may indicate that screw degradation does not compromise graft incorporation. Tendon Fixation-Femur Transfixion fixation. The semifix (Arthrex) and the bone

mulch screw (Arthrotek, Ontario, CA) are examples of transfixion fixation. There was no significant difference in failure or stiffness between the semifix and the EndoButton with Endotape (Smith and Nephew Endoscopy). In paired knees, there was no difference in failure between the bone mulch screw and the EndoButton; the bone mulch screw was slightly stiffer than the EndoButton, 24.4 N / m m and 21.2 N / m m , respectively (Table 5).43In cyclic biomechanical testing, both the transfixion (238 N / m m ) and the bone mulch screw (257 N / m m ) possessed superior stiffness to the EndoButton linked with either the Endotape (183 N / m m ) or the Endoloop (Ethicon, Inc, Somerville, NJ) (179 N/ram). The transfixion (1042 N) and the bone mulch screw (978 N) were stronger to failure than the

TABLE 5. Femoral Fixation Options for a Tendon Graft Construct

Failure (N)

Stiffness (N/mm)

Quadrupled ST/G with Semifix43

523 _ 263

34.2 ± 14.3

Quadrupled ST/G with Bone Mulch 43 Quadrupled ST/G with and EndoButton, Mersilene tape43 Quadrupled ST/G with EndoButton and Endotape43

583 520 618 663 678 699

24.4 34.8 22.4 18.1 20.6 30.2

Quadrupled ST/G with EndoButton and three #5 suture43 Quadrupled ST/G with EndoButton and 2 loops of Endotape4s Semitendinosus fixed with the EndoButton and tibial post76 Quadrupled ST/G with Mitek4a Quadrupled ST/G with interference screw with the RCI titanium screw59 Quadrupled ST/G with interference screw with BioScrew59 Quadrupled ST/G with interference screw with BioScrew, 1½mm graft sleeves4°

_+ 108 _+ 50 + 242 _+ 211 + 179 _+ 210

_+ 4.17 _+ 22.3 _+ 6.88 _+ 6.93 _+ 7.77 _+ 8.50

Mechanism of Failure Cross-pin toggled graft slipped off, tibial fixation failure Tibial fixation failure, implant failure Tape broke Tape broke, tibial fixation failure, tendon failure, implant pulled through bone

628 _+ 359

21.2 _+ 5.45

612 _+ 73 412 _+ 189 242 _+ 90.7

47 _+ 19 20.3 _+ 5.60 No stiffness reported

Implant pulled through bone, tibial fixation failure, suture failure, tendon failure Tibial fixation failure, implant pulled through the bone, tape broke No mode reported Implant pulled through bone Failed by graft slipping

341 _+ 162.9 530 -+ 186

No stiffness reported No stiffness reported

Failed by graft slipping Failed by graft slipping

NOTE: Data from biomechanical studies using human tissue. Abbreviations: ST/G, semitendinosus/gracilis. 260

BRAND ET AL

Endobutton linked with Endotape (644 N), but none failed at as high a level as the EndoButton linked with Endoloop (1342 N) (unpublished data). Both transfixion devices may allow independent tensioning of the 4 strands of the quadrupled hamstring tendon, which in the laboratory resulted in a statistically significant increase in failure strength of the quadrupled hamstring: graft and an 89% increase in stiffness. 72 A clinical study using cross-pin fixation in ACL reconstruction with a quadrupled hamstring tendon yielded comparable results to those reported in the literature. 73 Two patients had the pin repositioned after apparent migration. One of those patients and another patient later had the pin removed because of illiotibial band irritation. The head of that particular pin has subsequently been modified. EndoButton. In a comparative study, a hamstring construct fixed with an EndoButton and a tibial post failed at 612 N + 73 N compared with 416 N + 66 N in the patellar tendon group with interference fixation in young cadavers. 75 Stiffness did not vary significantly between both groups. Both constructs were only 20% to 30% of the failure strength of the native ACL, 2195 N + 427 N (Table 5). Brown et a143 tested the EndoButton with 5-0 suture, Mersilene tape (Ethicon, Inc, Somerville, NJ), and the Endotape (Table 6). Doubling the materials significantly increased their mechanical properties (Table 6). Direct biomechanical comparison between EndoButton linked with the Endoloop and linked with Endotape revealed similar stiffness and a much higher failure with the Endoloop, 1345 N versus 644 N for the Endotape-linked EndoButton (unpublished data). Mitek anchor. Brown et a143 compared the Mitek anchor directly to the EndoButton with the quadrupled hamstring graft in paired elderly human specimens. The EndoButton was significantly stronger (618 N v 412 N, P = .03), but stiffness was comparable in both groups, which would be expected with similar linkage materials (Table 5). The Mitek anchor failed by pulling through the bone. 43 Metal and biodegradable interference fixation. N o statisti-

significant difference was found between the BioScrew (341 N) and a titanium interference screw (242 N) in a human femoral model (Table 5).59In the same laboratory; it was found that the use of graft sleeves sized to within 0.5 m m of graft diameter improved ultimate failure strength significantly in the femoral tunnel to 530 N using the cal

TABLE 6. Linkage Material Properties Linkage Material

Stiffness (N/mm)

Mersilene tape, (Ethicon, Inc, Somerville, NJ) Doubled Mersilene Meadox, (Meadex Medical Inc, Oakland, NJ) Doubled Meadox

492+-28 873 -+ 45 509 + 5 2 1234 _+ 15

63.2_+7 163 _+ 14 37.8+12 109 _+ 2

Endotape, (Smith and Nephew Endoscopy, Inc, Andover, MA) Doubled Endotape Three #5 Ethibond sutures, (Ethicon, Inc)

699-+51 1520 _+ 89 801 + 5 9

63.9_+6 143 +_ 8 85.1 _+ 10

GRAFT FIXATION IN KNEE LIGAMENT SURGERY

FUTURE DIRECTIONS Douglas W. Jackson, MD, speaking in the Kennedy Lecturesnip at Speciality Day at the 66th Annual Meeting of the Academy of Orthopaedic Surgeons, emphasized the importance of outcome studies in meeting patient needs. Research in graft fixation has not yet come to this level. The relationship between less stiff and strong graft fixation, the interplay with rehabilitative efforts, and laxity of the reconstructed knee has not been established. Diminishing individual fiber movement within the tendon graft and the elimination of linkage materials will improve future soft tissue fixation. The combination of fixation devices, for example, use of a screw and washer and a biodegradable interference screw directly against a tendon graft--"hybrid fixation"--may be useful in the intermediate future. However, one is also combining the disadvantages of each device. In our preliminary biomechanical testing, strength and stiffness did not demonstrate quite the expected improvement. Biodegradable bone cement that allows for immediate fixation of the graft and eventual replacement with normal osseous tissue may be developed. 7 Graft fixation research has focused on the mechanical aspects at time zero. Research in Berlin suggests that compressive interference fixation may have different means of graft incorporation than traditional hamstring fixation, which is placed at a distance from the joint surface, n Manipulation of the biological environment may speed graft incorporation and rehabilitation. Fixation that allows immediate and secure fixation will aid rehabilitation, hasten return of muscle tone and force, and benefit patient outlook. At present, there is no strong clinical association between fixation that performs well in laboratory testing and objective knee stability. If this association is proven, the clinical association of laxity to clinical outcome and patient satisfaction can be investigated. Presently, few clinical studies are directed toward these issues. Comparative studies of different modes of graft fixation are important in this effort. Longitudinal studies, although difficult and fraught with methodological problems, will establish the link between patient satisfaction, residual laxity, and degenerative arthrosis.

CONCLUSIONS

Failure Load (N)

Data from Brown et al. 43

BioScrew as interference fixation (P K .05)15 (Table 5). The femoral ultimate failure of the BioScrew was statistically significantly nigher than a metal interference screw (266 N) in the same study. 61 As noted earlier, increasing screw length and screw diameter improved fixation, with screw length the more important of the two variables (unpublished data).

The amount of force required by ADLs and a progressive rehabilitation program has been estimated but not yet measured. Many forms of tendon fixation are placed at a distance from the joint and rely on linkage materials. Interference fixation meets several characteristics of ideal fixation. It is easy, reliable, achievable through a small incision, and can be placed close to the joint surface. Bioabsorbable interference fixation adds other elements of ideal fixation: it does not have to be removed, allows for

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postoperative magnetic resonance imaging, and consists of drill-through capabilities. Fixation strength is similar or better in most studies when biodegradable screws are used. Tibial fixation does not have as high a pull-out failure rate as femoral fixation when an interference screw is used with a bone plug or tendon. Although ultimate failure strength can be improved in a bone plug construct, most improvement will likely occur with tendon fixation. Biomechanical and biological data need to be correlated with clinical outcome studies.

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