Cross-Sectional Area Measurement of the Semitendinosus Tendonfor Anterior Cruciate Ligament Reconstruction

Masayuki Hamada, M.D., Konsei Shino, M.D., Tomoki Mitsuoka, M.D.,Nobuhiro Abe, M.D., and Shuji Horibe, M.D.

Summary: We measured the cross-sectional area (CSA) of the semitendinosustendon (SMT) in 79 anterior cruciate ligament (ACL)-injured patients usingmagnetic resonance imaging (MRI) to scrutinize their appropriateness for ACLgrafts. Measurements of the CSAs of the SMT with MRI were closely correlatedwith intraoperative direct measurement (g 5 0.697). The mean CSAs of the SMTmeasured with MRI ranged from 6.3 to 15.0 mm2 with a mean of 10.16 2.1 mm2.The CSA of the SMT measured with MRI proved to be a useful indicator todetermine preoperatively whether the SMT graft would be of adequate dimensions(7 mm or more in diameter, 60 mm or more in length) for ACL reconstruction. Ifthe CSA of the SMT was more than 11 mm2, a sufficiently thick and long graftcould be prepared with a tripled or quadrupled SMT in 89% of cases. We concludethat tissue CSA measurements using MRI could potentially be implemented as auseful tool for determining the most appropriate donor autograft tissue preopera-tively, thus minimizing harvest-site morbidity.Key Words: Anterior cruciateligament reconstruction—Semitendinosus tendon—Cross-sectional area measure-ment.

Anterior cruciate ligament (ACL) reconstructionusing the central third of the autogenous patellar

tendon (PT) or semitendinosus tendon (SMT) is fre-quently performed.1-7 Biomechanical studies of humangrafts for ACL reconstruction have shown that themaximum failure load of the central third of the PTbefore transplantation was 1.7 times greater than thatof the normal ACL, and that of the SMT, if useddoubled, could be estimated to 1.4 times greater thanthat of the normal ACL.8,9 After transplantation, how-ever, it is generally believed that the autogenous graftsdo not retain these maximum failure loads at time zero,because they undergo the process of ischemic necrosis,revascularization, and maturation. Several experimen-

tal studies have shown the maximum failure loadsdramatically reduce early after transplantation, andthen increase gradually over time to reach only 30% to50% of the contralateral ACL at 7 to 12 months aftertransplantation.10-14 Recently, a case report by Beyn-non et al.15 in human showed that the maximum failureload of the PT graft at 8 months after transplantationwas 87% that of the normal ACL. If it is taken intoconsideration that the maximum failure load of PTgraft before transplantation is 168% that of normalACL, the maximum failure load of the PT graft at 8months could be estimated to have reduced to approxi-mately 50% of that of the graft before transplanta-tion.8,9 Therefore, the maximum failure loads of suchintra-articularly transplanted grafts in humans, even ifthe process of maturation takes place, are inferior tothat of the normal ACL.

To compensate for this reduction in strength ofautogenous tissues after implantation, the use of graftsof equal or slightly larger diameter to that of thenormal ACL seems to be mandatory. In our clinic,

From the Department of Orthopaedic Sports Medicine, OsakaRosai Hospital, and the Department of Orthopaedic Surgery, OsakaUniversity Medical School (K.S.), Osaka, Japan.

Address correspondence and reprint requests to Masayuki Ha-mada, M.D., Department of Orthopaedic Sports Medicine, OsakaRosai Hospital, 1179-3 Nagasone-cho, Sakai, Osaka, 591, Japan.

r 1998 by the Arthroscopy Association of North America0749-8063/98/1407-1746$3.00/0

696 Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 14, No 7 (October), 1998: pp 696–701

when SMT is used for ACL reconstruction, the tendonwas tripled or quadrupled to obtain 7 mm or more graftdiameter, since the average diameter of the normalACL is estimated to be 7.5 mm.16 The graft should alsobe 60 mm or more in length, 25 to 30 mm of which arerequired to span the distance between the tibia andfemur, with 15 mm required for tendon to bone healingin the bony tunnels. Our recent 2-year follow-up withthis procedure showed excellent results in restoringknee stability and confirmed the adequacy of ourbaseline criterion in size of SMT graft (7 mm indiameter and 60 mm in length).16 However, it is notalways easy to harvest a graft that is sufficiently thickand long because of the large variability in size of theSMT.17 It is desirable to preoperatively measure thecross-sectional area (CSA) or dimensions of the SMT,to ensure that it is large enough for ACL graft. Thepurposes of this study were to evaluate (1) theprecision of magnetic resonance imaging (MRI) inmeasuring the CSA of autogenous SMT, (2) thecorrelation of body size with the CSA, and (3) thecorrelation of the CSA with length of the SMT. Inaddition, we attempted to preoperatively estimate thepossibility of harvesting a graft 7 mm or more indiameter, and 60 mm or more in length graft with theSMT.

MATERIALS AND METHODS

Seventy-nine patients who had undergone ACLreconstruction using autogenous SMT were studied.There were 41 male and 38 female patients with amean age of 23 years (range, 13 to 50 years). Theirmean height was 165 cm (range, 152 to 185 cm) andmean body weight was 65 kg (range, 49 to 115 kg).Preoperatively, axial T1-weighted MRI (repetitiontime, 300 msec, echo time 16 msec) were obtainedwith a GE 1.5-T imager (General Electric, Milwaukee,WI) using a circumferential, transmit-and-receive ex-tremity coil. A 12-cm field of view with a 2563 128matrix size was used. Slices were 5-mm thick with nointerslice gap for all studies. Using these axial images,CSAs of the SMT at the knee joint level weremeasured using an image analyzer linked to the MRsystem by counting the number of pixels with lowsignal intensity (Fig 1). The signal intensity of thetendon was less than 100, whereas that of the surround-ing tissues was above 400. Therefore, pixels withsignal intensity less than 100 were considered to beintratendinous, and pixels with signal intensity be-tween 100 and 200 were considered to constitute bothtendon portion and that of the surrounding tissues. The

number of pixels in the SMT was calculated by addingthe pixel number for the intratendinous portion andone half the pixel number for both tendon portion andthat of the surrounding tissues. The CSA of the SMTwas then calculated by multiplying the pixel numberby the single pixel area of 0.22 mm2.

To determine the accuracy of the measurements ofCSAs with MRI, the CSAs of SMT at the knee jointlevel (at 10 to 13 cm proximal from the distal tibialinsertion) were measured using a custom-made areamicrometer, which was similar to the one reported byHollis18 (Fig 2). The portion measured was determinedby the length of each SMT route from the distal tibialinsertion to the knee joint level when the knee wasextended. The CSA measurements were repeated atthree different locations, and the maximal value wasconsidered to be the CSAmeasured with area microme-ter. The tendon, harvested using a closed-loop tendonstripper (Acufex, Norwood, MA), was placed in theoval-shaped slot 3 mm in width. A constant pressure of0.12 MPa was then applied with an attached spring.19

The displacement of slide caliper was read, and thenthe CSA was calculated from this displacement.

After the intraoperative CSA measurement of theSMT, ACL grafts were prepared with similar methodsreported by Maeda et al.16 If the length of the tendonavailable for the ACL graft was greater than 24 cm, itwas quadrupled to create a graft a minimum of 6 cm inlength (n5 42). The length of the SMT in this study isnot the entire length, but length of effective portion forgrafting, i.e., the length of portion thick enough to be

FIGURE 1. Axial T1-weighted MRI of the knee. The SMT islabeled (arrow).

697CROSS-SECTIONAL AREA MEASUREMENT OF SMT

suitable for secure suturing by No. 2 or 3 polyestersutures. Otherwise, the tendon was tripled (n5 37).The diameter of the graft was then measured byAcufex sizing tubes. When the diameter was more than7 mm, the multiplied SMT alone was used for ACLreconstruction (group I, n5 41). If the diameter wasless than 7 mm, the ipsilateral gracilis tendon washarvested in the same manner as for the SMT anddoubled or tripled to reinforce the semitendinosusgraft (group II, n5 38).

Statistical analyses were conducted on Abacus Con-cepts, Stat View IV (Abacus Concepts, Inc, Berkeley,CA). Paired t test, unpairedt test, x-square test,Mann-Whitney test, and Pearson’s correlation coeffi-cient were performed to determine significance or todescribe the relationship, which was determined at theP , .05 level.

RESULTS

Precision of MRI MeasurementThe CSA of SMT measured with MRI ranged from

6.3 to 15.1 mm2 (average, 10.16 2.1 mm2), and thatmeasured with area micrometer ranged from 7.1 to17.0 mm2 (average, 11.26 2.1 mm2). Although, themeasured data with MRI was slightly smaller thanthose with area micrometer (P , .0001, pairedt test),these two measurements showed a close positivecorrelation (g 5 0.697,P , .0001) (Fig 3).

CSA of the SMT Versus Patients’ Body SizeThere was a slight correlation between the CSAs of

SMT measured with MRI and patient body size (bodyweight, g 5 0.292, P 5 .011; height, g 5 0.389,P 5 .0004) (Fig 4).

CSA Versus Length of the SMTThere was a slight correlation between the CSA

measured with MRI and the length of the SMT(g 5 0.246,P 5 .0285).

Length of the SMT Versus Patients’ Body SizeThere was a slight correlation between the length of

SMT and height (g 5 0.406,P 5 .0002), and betweenthe length of SMT and body weight (g 5 0.331,P 5 .0035).

SMT Graft Versus SMT and Gracilis TendonThere were no significant differences in height

(group I, 1676 7.9 cm; group II, 1646 7.3 cm) orbody weight (group I, 66.46 14.4 kg; group II,62.76 10.6 kg) between these two groups (height,P 5 .19; body weight,P 5 .21, unpairedt test) (Fig 5).Eighty percent (33 of 41 cases) of group I could bereconstructed with quadrupled SMT, and only 24% (9of 38 cases) of group II could be done with the graft.The percentages between these two groups werestatistically significant (P , .0001,x-square test). TheCSA of SMT in group I was 11.16 4.5 mm2 (range,7.7 to 15.1 mm2), and that in group II was 9.06 1.9

FIGURE 2. Custom-made areamicrometer used for intraopera-tive tendon measurement.

FIGURE 3. The CSA of the SMT as measured with MRI wasclosely correlated (g 5 0.697) with that directly measured using thearea micrometer.

698 M. HAMADA ET AL.

mm2 (range, 6.4 to 11.7 mm2), a statistically significantdifference (P , .0001, Mann-Whitney test) (Fig 5C).

DISCUSSION

Several studies have investigated the usefulness ofMRI for diagnosis of tendinous injuries, but very fewhave looked at tendon dimension measurements.20-26

Meisterling et al.27 evaluated morphological changesin the human PT using MRI after its central third hadbeen harvested for autogenous bone–patellar tendon–bone ACL reconstruction. However, they did not referto the precision of measurement with MRI. Thepresent study showed a close correlation between thedirect CSA measurement of the SMT and the CSA asdetermined by MRI, with the MRI estimation beingabout 10% smaller on average. Therefore, the MRIcould be used as a useful tool for evaluating theappropriateness of using the SMT for ACL autograft-ing.

Our results showed large variation in the CSA of theSMT, and the results were similar to those of Pagnaniet al.17 However, the relationship between the CSA ofSMT and body size has not been reported previously.Our study showed only a slight positive correlationbetween the CSA of SMT and body size. Therefore, ifwe surgeons reconstruct ACL in a uniform procedure,some patients whose SMT grafts are small for theirbody size or smaller than original ACL do exist. In thisrespect, we determined the minimum graft diameterfor ACL as 7 mm according to the value of CSA of

ACL reported by Noyes and Grood.8 Not all patientshave a sufficiently large SMT to meet this requirement,and the gracilis tendon is sometimes used to supple-ment the SMT. Some reports have shown that harvest-ing these hamstring tendons for ACL reconstructionresults in donor site morbidity. Marder et al.28 reportedsignificant weakness in peak hamstring torque. Morerecently, Nakajima et al.29 reported on patients whoshowed difficulty in knee flexion with the hip ex-tended. These reports suggest that harvesting SMT andgracilis tendon should be avoided, especially forhighly competitive athletes. Thus, it is desirable topreoperatively expect the necessity of adding anotherautogenous tissue other than SMT.

We have shown that body weight is not an appropri-ate indicator of SMT size. On the other hand, heightwas a fairly useful indicator, although there were nosignificant differences in height between group I(reconstruct ACL with SMT alone) and group II(reconstruct ACL with SMT and gracilis tendon). Asshown in Fig 5B, if the height of the patient was morethan 175 cm, 80% (8 of 10 cases) of cases wouldbelong to group I. If the height was less than 155 cm,only 17% (1 of 6 cases) of cases would belong to groupI. One explanation of this is that tall patients tend tohave a long enough SMT for quadrupling to 7 mmdiameter. The CSA of SMT was the most usefulindicator for the preoperative evaluation of the SMTgraft. As shown in Fig 5C, if the CSA of SMT wasmore than 11 mm2, 89% (24 of 27 cases) of the caseswere in group I. And if the CSA of SMT was less than

FIGURE 4. (A) The relationship between the CSA measured by MRI and body weight. (B) The relationship between the CSA measured byMRI and height.

699CROSS-SECTIONAL AREA MEASUREMENT OF SMT

8 mm2, only 25% (2 of 8 cases) of cases were ingroup I.

The CSA measurement in this study was performedusing routine knee images and no additional scanswere necessary. That is to say that, the routine MRIused to assess ligament and meniscus pathology canalso be used to assess the dimensions of prospectiveautogenous graft tissue. Using the CSA of the SMT,however, we could not always predict correctly whetherthe SMT was the composite long and thick enough forgrafting. Our results showed that a SMT graft longenough for quadrupling (more than 24 cm) had agreater chance to be a thick ACL graft. To preopera-tively estimate accurate dimension of the SMT, boththe length and the CSA might be evaluated. Naturally,

MR axial views of every 1 to 2 cm are required.However, this requires enormous scanning time andmuch longer installation time of the surface coil, andconsequently is more costly. Furthermore, it is not easyto determine the upper border of the tendon by MRI, asthe tendon transitionally change into muscle belly.Considering these factors, these extra axial scanningsdo not seem to be practical.

If size of the SMT was judged to be small preopera-tively, what kind of graft materials should be used? Toadd the gracilis tendon is a popular procedure. How-ever, if one wishes to avoid harvesting two flexortendons, and the PT is judged to be sufficient by thepreoperative MRI, then bone–patellar tendon–bonegraft may be a more suitable alternative. In some

FIGURE 5. Group I and group 2 compared. Group I, SMT alone("); group II, SMT plus gracilis (%). (A) Comparison betweengroup I and group II as a function of body weight. (B) Comparisonbetween group I and group II as a function of body height. (C) CSA ofSMT (group I versus group II).

700 M. HAMADA ET AL.

cases, allograft would be a suitable choice.30-31 Thus,when selecting an autogenous graft forACL reconstruc-tion, the CSA of autogenous tissues should be consid-ered to ensure a sufficient graft size and avoid potentialharvest site morbidity.

CONCLUSIONS

MRI is a useful tool for evaluating the CSA of theSMT preoperatively. The CSA of the SMT tendon is auseful indicator for estimating the final graft size forthe ACL reconstruction, if the quadrupled SMT ispreferred as a graft.

Acknowledgment: The authors thank Fumio Ohtani forhis expert assistance in the measurement of the cross-sectional area of the tendons.

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