Abstract Changes in the femoraland tibial bone tunnel were studiedprospectively after arthroscopic ACLreconstruction with quadruple ham-string autograft. To determinewhether tunnel enlargement can bedecreased by fixing the graft close tothe joint line having a stiffer fixationconstruct we compared “anatomical”(one absorbable interference screwfemorally, and bicortical fixationwith two absorbable interferencescrews tibially) and extracortical fix-ation techniques (Endobutton femo-rally, and two no. 6 Ethibond suturesover a suture washer tibially). Over a2-year period we evaluated 60 pa-tients clinically (IKDC scale, Cincin-nati Knee Score, KT-1000) andradiographically (confirmed byMRI). The operated knee was radio-graphed immediately postoperativelyand 6 and 24 months postoperatively.The femoral and tibial bone tunneldiameter was measured on antero-posterior and lateral images, and thetunnel area was calculated and com-pared to the initial area calculatedfrom the perioperative drill size. Inthe “anatomical” group the immedi-ately postoperative bone tunnel areawas 75% larger than the initial tun-nel area, after 6 months it was in-creased another 31%, and between 6and 24 months it remained basically

unchanged. In the “extracortical”group there was no significant en-largement immediately postopera-tively, but after 6 months it was 65%larger than the initial area of drilland graft size, and between 6 and 24 months it decreased to 47%.There was no correlation betweenthe amount of tunnel enlargementand clinical scores or KT-1000 mea-surement. Arthroscopic ACL recon-struction with quadruple hamstringautograft is associated with bone tun-nel enlargement. Using a purely ex-tracortical fixation technique thussignificantly increased the tibial andfemoral tunnel area during the first 6 postoperative months, while it de-creased slightly thereafter. The inser-tion of large interference screws ap-parently not only compresses thegraft in the bone tunnel but also sig-nificantly enlarges the bone tunnel it-self. The immediate enlargement atthe time of the operation is followedby a reduced further enlargement at6 months and then stabilization. Tun-nel widening did not influence clini-cal outcome over a 2-year period.

Keywords Anatomical fixation ·Anterior cruciate ligament reconstruction · Bone tunnel · Extracortical fixation · Hamstringautograft

KNEEKnee Surg, Sports Traumatol, Arthrosc(2002) 10 :80–85

DOI 10.1007/s00167-001-0267-6

J.-U. BuelowR. SieboldA. Ellermann

A prospective evaluation of tunnel enlargement in anterior cruciate ligament reconstruction with hamstrings: extracortical versus anatomical fixation

Received: 2 August 2001Accepted: 18 November 2001Published online: 27 February 2002© Springer-Verlag 2002

J.-U. Buelow (✉ )64 View Terrace, East Fremantle 6158, Western Australiae-mail: Buelows@hotmail.com, Tel.: 0061-8-93198363, Fax: 0061-8-93198380

R. Siebold · A. EllermannARCUS Sportsclinic, Wilhelm-Beckerstrasse 15, 75179 Pforzheim, Germany

Introduction

Hamstring tendons are being used increasingly for recon-struction of the anterior cruciate ligament (ACL). Thesehave a relatively low donor site morbidity, the dimensionof the graft is more comparable to that of the intact ACL,the ultimate strength is as high as 4108 N, and a multiplebundle reconstruction is possible. On the other hand, thereis still concern over tendon healing within the osseoustunnels. Significant tunnel enlargement has been reportedin hamstring ACL reconstruction versus patellar tendonautograft [3, 13]. Webster et al. [19] demonstrated morefrequent and greater femoral bone tunnel enlargement fol-lowing ACL reconstruction with hamstring grafts than inpatellar tendon grafts. The proposed mechanisms of tun-nel enlargement are mechanical (bungee cord, wind-shield-wiper effect) and biological (synovial fluid) result-ing in osteolysis of the tunnel [3, 5, 9, 13, 18].

Clatworthy et al. [3] presented the results of a prospec-tive multicenter study comparing four different fixationtechniques for ACL reconstruction with quadruple ham-string graft. Two “anatomical” fixation techniques (bioab-sorbable interference screw, RCI screw) were comparedto a fixation technique with bone mulch screws and apurely extracortical fixation technique (Endobutton/suturewasher). Surprisingly, the most tunnel enlargement wasseen with anatomical fixation. On the other hand, Weileret al. [20] showed that using an interference screw forhamstrings leads to direct tendon-to-bone healing withoutfibrous interzone occurs, Ishibashi et al. [10] reported thebiomechanical advantage of an anatomical interferencescrew fixation. So far no clinical correlation between tun-nel enlargement and poor clinical result has been shown[5, 12, 16, 19], but it is at least clear that a wide bone tun-nel filled with fibrous tissue can be difficult to treat incases of revision surgery.

The purpose of our study was prospectively to evaluatechanges in the bone tunnel area following arthroscopicACL reconstruction with quadruple-hamstring autograft.Our hypotheses was that tunnel enlargement can be de-creased by fixing the graft close to the joint line having astiffer fixation construct. Therefore we compared an“anatomical” fixation with interference screws to an ex-tracortical fixation technique. We used radiography se-quentially to monitor the time course of these changesover a 2-year period and examined the correlation of re-sults to clinical parameters. Magnetic resonance imaging(MRI) were performed to confirm the radiography mea-surements.

Methods and materials

Study group

The study included 60 patients undergoing arthroscopic ACL re-construction with quadruple hamstring autograft. These were di-

vided into two groups of 30 each. Those treated by the “anatomi-cal” fixation technique included 18 men and 12 women (18 rightside, 12 left) with a mean age of 33.1 years (range 18–45) whilethose in the “extracortical” group included 17 men and 13 women(16 right side, 14 left) with a mean age of 30.9 years (range17–44). The inclusion criteria for the study were: (a) unilateral an-terior laxity confirmed clinically and by MRI, (b) no previous kneeligament surgery, (c) a normal contralateral knee, and (d) the will-ingness to follow the study protocol for at least 2 years. Patientswith additional knee ligament injuries, arthritic changes, or mal-alignment were excluded. When patients gave their informed con-sent and were accepted into the study, we performed a preopera-tive assessment including history, clinical examination, objectiveknee laxity test with the KT-1000 [4], and radiography. Patientswere evaluated according to International Knee DocumentationCommittee (IKDC) [7] and Cincinnati Knee Score [15].

Operative technique

Arthroscopic ACL reconstruction using a four-strand semitendi-nous and gracilis autograft was performed by a senior surgeon. Pa-tients were operated on in a sequential series. The surgery was per-formed between September 1997 and February 1998.

Surgical technique

The semitendinosus-gracilis tendon was harvested through an an-terior medial longitudinal tibial incision at the pes anserinus, and alength of tendon, usually about 24 cm long, was acquired. An ade-quate intercondylar notchplasty was performed. Once prepared,the graft diameter was measured, and the tunnels drilled accord-ingly. The tibial tunnel was drilled at a 40–50° angle to the tibialplateau, and the femoral tunnel was drilled about 3–5 mm to theposterior rim depending on graft size.

“Anatomical” fixation

After positioning of the graft the femoral bioabsorbable interfer-ence screw was inserted line-to-line with the tunnel over a guidewire. We used 23-mm poly-L-lactic acid (PLA) Bioabsorbable In-terference Screws (Arthrex, Karlsfeld, Germany) available in di-ameters of 7, 8, 9, and 10 mm. At the tibia the graft was fixed attwo cortical locations (“bicortical”) within the tibial tunnel usingtwo bioabsorbable interference screws: the proximal tibial bioab-sorbable interference screw was placed anatomically at the jointline anterior to the graft, and the distal bioabsorbable interferencescrew was placed posterior to the graft at the cortical opening ef-fectively crosslocking the tendons within the tunnel.

Extracortical fixation

Fixation to the Endobutton was achieved using a triple no. 6 Ethi-bond suture with an average length of 25 mm (20–35 mm). Thefour free ends of the hamstring-tendons were each armed with no.6 Ethibond sutures using a whip-stich technique and were tied overa suture washer. A tight fit of the graft in the bone tunnel wasaimed for in all patients. The femoral and tibial tunnel diameterwas documented at time of surgery.

Rehabilitation

The postoperative rehabilitation protocol was similar in all pa-tients. A continuous passive motion machine was used for the first2 days. Patients were discharged from the hospital on day 2. Post-

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operative full extension was easily achieved, and full weight bear-ing was allowed within the first 2 weeks. No brace was used. Pa-tients attended outpatient physical therapy one to three times perweek for a period of 6–12 weeks. Rehabilitation included station-ary bicycling after 2 weeks and jogging 3 months postoperatively.Closed-chain exercises were encouraged immediately postopera-tively, and open-chain exercises were allowed at 3 months. Fullsports activity was allowed 1 year postoperatively.

Clinical evaluation at follow-up

Patients were radiographed immediately postoperatively (on thesecond postoperative day). All 60 patients were available for thefollow-up at 6 months, and 58 were available for the follow-up at 24 months postoperatively (30 “anatomical”, 28 “extracortical”;two patients had reruptured their ACL during sports 7 and 11 monthspostoperatively). The average 6-month follow-up was at 6.6 months,and the average 24-month follow-up at 26.6 months in the“anatomical” group, and 6.9 and 27.1 months, respectively, in the“extracortical” groups. Evaluation consisted of history, clinical ex-amination, IKDC test [4], and Cincinnati Knee Score [15]. Objec-tive knee laxity was measured by the KT-1000 [4].

Radiographic evaluation at follow-up

Radiography (anteroposterior and lateral weight bearing in exten-sion) of the reconstructed knees was performed immediately post-operatively (on the second postoperative day) and 6 and 24 monthspostoperatively.

The margins of the bone tunnels (Fig.1) could be identified asa thin line on the immediately postoperative radiography and wereeasy to identify as a line of cortical bone at 6 and 24 months post-operatively. The femoral tunnel diameter was measured 1 cmproximal to the ACL origin and the tibial diameter 1 cm distal tothe ACL insertion, both in the sagittal and coronal plane. A 10-cmmarker was taped to the lateral aspect of the leg to determine themagnification factor.

The length of the bone tunnels was measured on lateral radi-ography. Femoral tunnel placement was documented by measuringthe distance from the posterior aspect of Blumensaat’s line to theposterior aspect of the femoral tunnel. Tibial tunnel placement wasmeasured according to Howell and Barad [8]. The shape of thetunnels were classified as either cone type, line type or cavity typeas described by Peyarche et al. [16].

MRI evaluation

MRI was performed on the same day as radiography in each pa-tient to validate the tunnel measurements on the latter. The diame-ter of the bone tunnel was measured on the sagittal and coronalplanes and was compared to the measurements on plain radiogra-phy. A 0.2-T Magnetom (Arthroscan) was used for the examina-tion.

Tunnel widening calculation

At time of follow-up (immediately postoperative, 6 and 24 months)the anteroposterior and lateral bone tunnel diameters were mea-sured and adjusted for magnification. The tibial and femoral tunnelareas were calculated and averaged, and the result was comparedto the initial tibial and femoral tunnel areas calculated according tothe perioperative drill size. (a) We expressed the change in “bonetunnel area” as a percentage. Area was chosen over width as thetunnels are three dimensional structures. The percentage change inarea was chosen over the absolute change in area. (b) We calcu-lated tunnel widening as: (follow-up area – initial area)/initial area.

“Tunnel widening” was defined as a greater than 50% increasein the femoral or/and the tibial tunnel area.

Statistics

Student’s t test was used to compare the amount of tunnel enlarge-ment between time intervals. Pearson’s correlation coefficient was

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Fig.1a, b Femoral and tibialtunnel margins 6 months post-operatively. a Anteroposteriorradiography. b Lateral radiog-raphy

used to characterize the relationship of tunnel enlargement to clin-ical parameters and tunnel placement. Statistical significance wasaccepted at the 95% confidence level (P<0.05).

Results

Radiographic results

“Anatomical” group

Immediately postoperative radiographs showed that thefemoral and tibial bone tunnel area were 75% greater thanthe initial area of drill and graft size at the time of the op-eration (P<0.05). After 6 months the area had increasedanother 31%, but between 6 and 24 months it remainedbasically unchanged (Table 1). Comparing the immediatelypostoperative radiographs with those at 6 months showedthat tunnel widening was 6% on the femoral side and 0%on the tibial side. Between 6 and 24 months the measure-ments remained basically unchanged (Table 2).

“Extracortical” group

The immediately postoperative radiographs showed noenlargement of the femoral and tibial bone tunnel area

over the initial area of drill and graft size. After 6 monthsthe average bone tunnel was 65% larger than the initialtunnel area (P<0.05). Between 6 and 24 months the aver-age area decreased slightly to 47% (Table 1). The inci-dence of tunnel widening was 80% on the femoral and47% on the tibial side 6 months postoperatively. At 24 months the incidence of tunnel widening decreasedslightly to 76% femoral and 45% tibial (Table 2).

There was no significant difference in the placement ofthe femoral and tibial tunnels. The femoral bone tunnelswere an average of 1.1 mm (range 0–4) anterior to theposterior aspect of Blumensaat’s line in the “anatomical”group and 1.16 mm (0–2) in the “extracortical” group.Corresponding figures for the tibial bone tunnels were1.16 mm (–6 to +5) and 1.56 mm (–2 to +6). Evaluationof tunnel shape revealed largely equivalent expansion ofthe tunnels throughout their length in the two groups. Theline type was seen in 83% (Table 3). the femoral/tibialtunnel lengths were 49.7/33.9 mm in the “anatomical”group and 48.9/33.5 mm in the “extracortical” group.

MRI results

MRI performed at 6 and 24 months confirmed the radio-graphic measurements; there was no significant differencein the tunnel diameter between MRI measurement andthat obtained from anteroposterior and lateral radiogra-phy. The average difference was 0.3 mm, and the varia-tion ranged from the MRI diameter being 0.5 mm lessthan the radiography to 0.6 mm greater. No sign of osteol-ysis was seen around the PLA screws in the “anatomical”group. None of the screws were absorbed or showed signsof partial absorption at 2-year follow-up.

Clinical results

Table 4 summarizes the results of the IKDC, CincinnatiKnee Score and KT-1000. There was no significant differ-ence in clinical outcome between the two groups. Therewas also no significant correlation (P>0.05) between theamount and incidence of tunnel widening and the clinicalresults or the KT-1000 measurements.

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Table 1 Tunnel enlargement 2days and 6 and 24 monthspostoperatively (percentage in-crease in area)

“Anatomical” fixation Extracortical fixation

2 days 6 months 24 months 2 days 6 months 24 months

Femoral 78 119 117 0 74 63Tibial 73 94 94 0 56 49Mean 75 106 104 0 65 47

Table 2 “Tunnel widening” (=greater than 50% increase in tunnelarea), comparison of immediately postoperative area with the areaafter 6 and 24 months

“Anatomical” fixation Extracortical fixation

6 months 24 months 6 months 24 months

Femoral tunnel 6% 6% 80% 76%Tibial tunnel 0% 0% 47% 45%

Table 3 Tunnel shape (at 6/24 months postoperatively)

“Anatomical” fixation Extracortical fixation (n=30/30) (n=30/28)

Femoral Tibial Femoral Tibial

Line 20 29 24 (22) 27 (25)Cavity 7 1 2 2Cone 3 0 4 1

Discussion

Arthroscopic ACL reconstruction with four-strand semi-tendinosus and gracilis graft is associated with significantfemoral and tibial tunnel enlargement. In contrast to ourhypothesis, the average enlargement of the femoral andtibial bone tunnels was 106% with “anatomical” interfer-ence screw fixation and 65% with the “extracortical” fix-ation technique 6 months postoperatively. Thereafter therewas stabilization with no significant change in tunnel areaup to 2 years postoperatively in both groups.

The insertion of an interference screw apparently notonly compresses the graft in the tunnel but also leads to anenlargement of the bone tunnel itself. This finding ex-plains the findings by previous investigators [3] of a gen-erally larger postoperative bone tunnel area when interfer-ence screws are used than with purely extracortical fix-ation. Taking this increase in tunnel area into considera-tion, the incidence of tunnel widening could be reducedsignificantly in this group compared to that with the ex-tracortical fixation. The incidence of tibial tunnel widen-ing at 6 months was 0% in the “anatomical” group and47% in the “extracortical” group. On the femoral side therespective figures were 6% and 80%. During the first 6 months the average tunnel enlargement was 31% in the“anatomical” group and 65% in the “extracortical” group.In neither group was there further tunnel enlargement af-ter 6 months.

Between 6 and 24 months postoperatively there waseven a slight, but not significant, decrease in tunnel areain the “extracortical” group. This finding is comparable tothe results of previous investigators [16, 19]. Peyrache etal. [16] radiographed 44 patients 3–36 months after ACLreconstruction with autogenous bone patella-tendon bonegraft and reported that tunnel enlargement was evident at3 months, did not change significantly up to 2 years, andthen decreased.

Retrospectively, serial MRI or computed tomographyimages would have been a better tool to measure the tun-nel enlargement in the early postoperative period up to 3 months. Fink et al. [6] showed that computed tomogra-phy is superior to plain radiography on the tibial side, es-pecially in the early postoperative phase, when the tunnel

margins are sometimes not easy to identify on radiogra-phy. In our study MRI was performed in all patients at 6and 24 months. The MRI findings confirmed our plainfilm measurements but did not add significant informationor accuracy.

There are different theories to explain the phenomenonof tunnel enlargement. Motion of the graft in the bone tun-nel (“bungy-cord effect,” “windshield-wiper effect”) mayplay an influential role, as well as biological components[3, 5, 9, 13, 18].

Our study hypothesis was that graft tunnel motionwould be eliminated by anatomical fixation with interfer-ence screws, leading to direct tendon-to-bone healing with-out tunnel widening. Furthermore, if there was a “wind-shield-wiper” or “bungy-cord” effect, cone-shaped tunnelswould be expected in the extracortical fixation group. Ourevaluation demonstrated essentially equivalent expansionthroughout the length of the tunnels in both groups with-out a significant difference between the groups. This find-ing supports the theory of a biological component to tun-nel enlargement. Synovial fluid in the bone tunnel mayplay an important role [3, 5]. Cytokines affecting bone re-sorption have been identified in normal synovial fluid [11,14, 17, 19] and following ACL disruption [1, 2].

Weiler et al. [20] showed in histological studies (sheepmodel) that using an interference screw for hamstringsleads to a direct tendon-to-bone healing without a fibrousinterzone. This suggests that no tunnel widening wouldoccur when an interference screw is used for the fixationof hamstrings in a bone tunnel. In accordance with thesefindings, the incidence of tunnel widening was signifi-cantly less in the “anatomical” group than in the “extra-cortical” group. The fact that there was still an averagetunnel enlargement of 31% in the first 6 months in the“anatomical” group cannot be fully explained. A combi-nation of micromotion of the graft and a biological com-ponent is likely.

In conclusion, tunnel enlargement does not appear toaffect the short-term efficacy of an ACL reconstruction.We observed no effect of tunnel enlargement on clinicaloutcome or objective laxity (measured by KT-1000) overa 2-year period. These findings are supported by severalother investigators [5, 12, 16, 19]. Inserting an interfer-ence screw created an average of 75% of tibial and femoral

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Table 4 Clinical scores andKT-1000 findings “Anatomical” fixation Extracortical fixation

Preoperative 24 months Preoperative 24 months (n=30) (n=30) (n=30) (n=28)

IKDC normal (%) 0 17 0 21Nearly normal (%) 0 77 0 68Abnormal (%) 33 3 34 11Severely abnormal (%) 67 3 66 0Cincinnati Knee Score 46±10.2 86±8.5 44±9.8 87±8.9KT-1000 side-to-side difference (mm) 6.2 1.8 6.3 1.95

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tunnel enlargement. This finding explains the generallylarger postoperative tunnel area when interference screwsare used than with purely extracortical fixation. Postoper-ative serial MRI or computed tomography starting on thefirst postoperative day is necessary to provide exact an-swers, but it seems that further tunnel enlargement can bereduced by using interference screws rather than extracor-tical fixation techniques. The interference screws may re-

duce micromotion and synovial fluid at the graft-bone in-terface and allow a direct tendon to bone healing withouta fibrous interzone. On the other hand, the total tunnelarea and “bone defect” is larger in the case of interferencescrews. None of the screws were absorbed at 2-year fol-low-up. The long-term effects of these findings must stillbe determined.

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