SCIENTIFIC ARTICLE
MRI of double-bundle ACL reconstruction: evaluation of graft findings
Tommi Kiekara&Timo Järvelä&Heini Huhtala&
Antti Paakkala
Received: 15 June 2011 / Revised: 25 August 2011 / Accepted: 12 September 2011 / Published online: 30 September 2011
#ISS 2011
Abstract
ObjectiveTo demonstrate the magnetic resonance imaging (MRI) findings of double-bundle (DB) anterior cruciate ligament (ACL) reconstruction grafts.
Materials and methodsSixty-six patients with DB ACL reconstruction were evaluated with MRI 2 years postoper-atively. Graft thickness was measured separately by two musculoskeletal radiologists. The MRI findings of graft disruption, signal intensity (SI) changes, cystic degenera-tion, arthrofibrosis, and impingement were analyzed. The statistical significance of the association between MRI findings was calculated.
ResultsThe mean anteromedial (AM) graft thickness was reduced 9% and the mean posterolateral (PL) graft thickness was reduced 18% from the original graft thickness. Disruption was seen in 3% of AM grafts and 6% of PL grafts and a partial tear in 8 and 23%, respectively. Both grafts were disrupted in 3% of patients.
Increased SI was seen in 14% of intact AM grafts and in 60% of partially torn AM grafts (p=0.032). In PL grafts the
increased SI was seen in 51% of the intact grafts and in 93% of the partially torn grafts (p=0.005). Cystic degeneration was seen in 8% of AM grafts and in 5% of PL grafts. Diffuse arthrofibrosis was seen in 5% of patients and a localized cyclops lesion in 3% of patients. Impingement of the AM graft was seen in 8% of patients.
Conclusion Both grafts were disrupted in 3% of patients.
Also, the frequencies of other complications were low. The use of orthogonal sequences in the evaluation of the PL graft SI seems to cause volume-averaging artefacts.
Keywords MRI . Anterior cruciate ligament . Double-bundle reconstruction
Introduction
The anatomy of the anterior cruciate ligament (ACL) consists of two bundles: anteromedial (AM) and posterolateral (PL).
The bundles are named according to their insertion on the tibia.
The AM bundle originates more proximally and anteriorly on the lateral femoral condyle than the PL bundle [1–3]. On the tibial site the AM bundle inserts anteromedially and the PL bundle posterolaterally [1–3]. When the knee is extended, the PL bundle is tight and the AM bundle is relatively loose. As the knee is flexed, the AM bundle tightens and the PL bundle loosens. The PL bundle is also tightened during internal and external rotation of the tibia [4–6].
Some studies have shown that the DB (double-bundle) ACL reconstruction restores knee kinematics and especially rotational stability of the knee more closely than the SB (single-bundle) ACL reconstruction, although anterior stability of the knee can be restored well with the SB ACL reconstruction, too [4, 7–16]. Modern surgical T. Kiekara (*):A. Paakkala
Medical Imaging Centre, Tampere University Hospital, FIN-33521 Tampere, Finland
e-mail: tommi.kiekara@pshp.fi A. Paakkala
e-mail: antti.paakkala@pshp.fi T. Järvelä
Sports Clinic and Hospital Mehiläinen, FIN-33210 Tampere, Finland
e-mail: Timo.Jarvela@sci.fi H. Huhtala
School of Health Sciences, University of Tampere, FIN-33014 Tampere, Finland
Skeletal Radiol (2012) 41:835–842 DOI 10.1007/s00256-011-1285-1
and two femoral tunnels and various tendon autografts and fixation devices [4, 7–16]. According to some prospective, randomized clinical studies it seems that DB ACL reconstruc-tion results in better rotareconstruc-tional stability and fewer graft failures when compared to SB ACL reconstruction [9,10,14,16,17].
MRI is the preferred imaging modality for the evaluation of ACL graft reconstructions [18, 19]. Prior studies have described the appearance and maturation of an intact SB ACL reconstruction graft [20, 21] and also the MRI findings of postsurgical complications [18, 19, 21–24].
The most common complications include graft rupture, cystic degeneration of the graft, graft impingement, and arthrofibrosis [18,19,21–24].
According to Casagranda et al. [25] the complications and MRI findings of DB ACL graft reconstruction are similar to those of SB reconstruction. The normal appearance of DB grafts in MRI with increased proton density (PD)-weighted SI at 6 months after operation and decreased SI at 12 months after operation is described by Poellinger et al. [26]. A preliminary study investigating the T2-weighted SI of DB ACL grafts 1 year after operation suggests that the PL graft SI is increased at that time compared to the AM graft SI [27].
In SB ACL reconstruction the graft SI is increased in both T1-weighted and T2-weighted images during the ligamenti-zation period 3–24 months after surgery [20,28].
To date, there is only one prospective study [27] of the DB ACL reconstruction graft MRI SI findings. Because the ligamentization may cause increased graft SI up to 2 years after operation we aimed to perform the MRI after the ligamentization period. In this study we aim to systematically report the MRI findings of both normal graft and complica-tions including disruption, cystic degeneration, arthrofibrosis, and impingement of the DB ACL reconstruction graft.
Materials and methods
Patients
Between 2004 and 2008, 75 patients underwent DB ACL reconstruction carried out by a single experienced
ortho-paedic surgeon. The inclusion criteria were primary ACL reconstruction and closed growth plates. The anatomical DB technique used in this study and described previously by Järvelä [9] uses AM portal and doubled semitendinosus and doubled gracilis autografts with a bioabsorbable interference screw fixation. Eight patients were lost to follow-up and one patient underwent revision ACL recon-struction. Finally, 66 patients (49 male and 17 female) were available for 2-year MRI evaluation. Mean patient age was 35 years (range 19–59 years). The research was approved by the medical ethics committee at the study hospital.
Written consent was required.
MRI analysis
MRI was performed at a mean 22 months (range 16–29 months) postoperatively with a 1.5-T Signa Excite HD imager (GE Medical Systems, Milwaukee, WI) using an eight-channel receiver/transmitter extrem-ity coil. The knee was placed into the coil in a slight flexion of a mean 8° resulting from the shape of the coil and imaged at rest. The imaging planes were chosen the same way in all patients. The MRI sequences and parameters are shown in Table 1.
The MR images were evaluated by two experienced musculoskeletal radiologists who were unaware of the patients’clinical data. Evaluations were done on an Impax DS 3000 workstation (Agfa HealthCare, Mortsel, Belgium).
Both radiologists measured the thickness of the grafts separately, and the interobserver agreements were evaluat-ed. The mean values of these measurements were usevaluat-ed.
Other MRI findings were evaluated together with agree-ment by consensus.
Graft thickness was measured for both grafts anteropos-teriorly (AP) and mediolaterally (ML) at a distance of 5 mm above the tibial plateau (Fig.1). Disrupted grafts could not be reliably measured and were excluded from graft measurements. The original thickness of both grafts was measured at the operation by pulling each graft in a tension through the specific metallic tubes made for the measure-ments. The thickness of the graft was the same as the Table 1 MRI sequences and
their parameters
SagSagittal,Corcoronal,Ax axial,Obloblique,FSEfast spin echo,FSfat saturation,TRtime to repeat,TEtime to echo,NEX number of excitations,FOV field of view,ETLecho train
TR (ms) TE (ms) NEX Slice/gap (mm/mm) Matrix FOV (cm) ETL
Sag T1 FSE 500 16 2 4.0/1.0 512/224 20 2
Sag PD FSE 2,320 24 2 4.0/1.0 512/256 20 8
Sag T2 FSE 3,740 78 2 4.0/1.0 512/224 20 16
Cor T1 FSE 500 16 2 4.0/1.0 512/224 20 2
Cor T2 FSE FS 3,300 74 2 4.0/1.0 256/224 20 16
Ax PD FSE FS 1,940 40 2 4.0/1.0 256/224 18 10
Obl Sag T1 FSE 660 16 2 3.0/0.3 512/224 20 2
Obl Cor T1 FSE 660 16 2 3.0/0.3 512/224 20 2
836 Skeletal Radiol (2012) 41:835–842
diameter of the metallic tube the graft was able to pass through [9].
Graft disruption was assessed. A graft was considered disrupted when no intact fibers were seen and fluid signal was interposed between the ends of graft fibers [18,19,22, 24]. The graft was considered partially torn when a focal graft thinning compared with any detected segment of normal graft diameter was seen [22].
The SI of the intra-articular portion of both grafts was analyzed as described by Howell [28]. The intra-articular portion of the grafts was divided into proximal, middle, and distal thirds (Fig.2). The SI was analyzed on PD-weighted and T2-weighted images and graded on a scale with (I) being a normal SI similar to posterior cruciate ligament (PCL), (II)
>50% of the graft having a normal SI, and (III) <50% of the graft having a normal SI. The grade IV by Howell (100% of the graft having an increased SI) was incorporated with grade III. When increased PD-weighted and T2-weighted SI was present, T1-weighted SI changes were also analyzed.
Cystic degeneration of the graft was defined as a fluid collection within the graft [19]. The cyst was located within the portion of the graft inside the femoral tunnel, in the intra-articular portion of the graft, or inside the tibial tunnel.
Roof impingement of the graft was defined as contact of the impinged graft with the anteroinferior margin of the intercondylar roof and posterior bowing and SI alteration of the graft [19,29]. Osteophytes of the femoral intercondylar notch were assessed.
In addition, generalized and localized arthrofibrosis was
tissue in the knee joint [19, 24]. Localized anterior arhtrofibrosis, or cyclops lesion, was defined as a nodular fibrous lesion in the anterior intercondylar notch [24].
Finally, PCL, lateral collateral ligament (LCL), medial collateral ligament (MCL), quadriceps tendon and patellar tendon were analyzed and graded as normal, degenerative and thickened, partially torn, or torn.
Data analysis
The data analysis was carried out using SPSS 14.0 statistical software. Interobserver agreements between the radiologists’measurements were estimated according to the method of Bland and Altman [30] using Strata 8.2 statistical software. The statistical significance of associa-tion between MRI findings was calculated using Fisher’s Exact Test except for the equality of means in which the two-tailedt-test was used.
Results
The findings of the graft evaluation are shown in Table2.
Graft thickness
The mean original graft thickness (femoral drill size) in the operation for the AM graft was 6.4 mm (range 6.0–7.5 mm) and 5.9 mm (range 4.0–6.0 mm) for the PL graft. In the Fig. 2 Evaluation of the AM graft SI (sag PD). The SI of the proximal third is normal. The SI of the middle third is grade III, and the distal third grade II
Fig. 1 Anteroposterior graft thickness was measured by evaluating a sagittal PD-weighted image. The SI of both grafts is normal
Skeletal Radiol (2012) 41:835–842 837
2.0–8.0 mm) and the ML thickness 5.9 mm (range 3.0–
9.0 mm). The mean PL graft AP thickness was 4.6 mm (range 2.0–7.0 mm) and the ML thickness was 5.1 mm (range 2.0–8.0 mm). Compared with the mean original graft thickness, the AM graft had reduced 9% and the PL graft 18% in diameter. The interobserver agreement on measure-ments of the graft thickness between the two radiologists was excellent (Table3).
Graft disruption
Of the 66 patients included in this study, 5 patients had a partially torn AM graft and 2 had a disrupted AM graft. The PL graft was partially torn in 15 and disrupted in 4 patients.
Both AM and PL grafts were disrupted in 2 patients (Fig.3). The frequencies of disruptions and partial ruptures are displayed in Table4.
Graft SI findings
AM graft SI was graded normal in 53 patients, grade II in 6 patients, and grade III in 5 patients. The intra-articular part
of the graft was divided into proximal, middle, and distal parts (Fig.2). The increased AM graft SI was seen in the proximal part in 9 patients, in the middle part in 11 patients, and in the distal part in 7 patients (Table5).
PL graft SI was graded normal in 24 patients, grade II in 20 patients, and grade III in 18 patients. The increased PL graft SI was seen in the proximal part in 32 patients, in the middle part in 36 patients, and in the distal part in 23 patients (Table 5).
Altogether, SI was increased in 17% of AM grafts and in 61% of PL grafts. All of the grafts with increased PD-weighted and T2-weighted SI also had increased T1-weighted SI.
Cystic degeneration
Cystic degeneration with fluid signal within the graft was seen in five AM grafts and in three PL grafts (Fig.4). Cysts Table 2 The findings of graft evaluation
Graft
AM (n) PL (n)
Graft thickness (mean) mm AP 5.8 4.6
ML 5.9 5.1
Graft disruption Partial tear 5 15
Disrupted 2 4
Graft SI findings Normal 53 24
Grade II 6 20
Grade III 5 18
Cystic degeneration 5 3
Arthrofibrosis (n) 3
Cyclops lesion (n) 2
Impingement (n) 5
AMAnteromedial, PLposterolateral,APanteroposterior,ML medio-lateral,SIsignal intensity
Table 3 Bland-Altman comparison of graft thickness measurements between the two radiologists
Mean difference (mm) Limits of agreement (mm)
AM graft AP −0.094 −1.41 1.23
AM graft ML 0.032 −1.17 1.23
PL graft AP 0.063 −1.44 1.57
PL graft ML −0.081 −1.40 1.24
AM Anteromedial, PL posterolateral, AP anteroposterior, ML
Fig. 3 MRI of arthrofibrosis and disruption of both grafts (sag PD).
Thearrowindicates the remnant of the AM graft. No PL graft present
Table 4 Frequencies of the different rupture patterns for the anteromedial (AM) and posterolateral (PL) graft
AM graft PL graft
Disrupted Partial rupture Intact Total
Disrupted 2 0 0 2
Partial rupture 1 4 0 5
Intact 1 11 47 59
Total 4 15 47 66
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were seen in the part of the graft inside the tibial tunnel in four AM grafts and in two PL grafts. Other locations were in the part of the graft inside the femoral tunnel (one AM graft cyst) and in the part of the graft inside the joint space (two PL graft cysts).
Arthrofibrosis
Diffuse arthrofibrosis was seen in three patients and a localized cyclops lesion in two patients (Fig.3).
Impingement
Graft impingement with posterior bowing and increased SI in AM graft was seen in five patients. None of the patients had PL graft impingement. Intercondylar osteophytes were seen in five patients.
Associated ligament findings
All patients had a normal PCL and a normal patellar tendon. A partial rupture of LCL was seen in three patients and a thickened MCL and a thickened quadriceps tendon were both seen in one patient. No associated ligament tears were found preoperatively or at the operation with these patients.
Association between MRI findings
The thickness of the grafts with normal SI or increased SI was evaluated. The mean AP thickness of AM grafts with normal SI was 5.9 mm and for grafts with increased SI 5.3 mm (p=0.046). Similarly, the mean AP thickness of PL grafts with a normal SI was 5.1 mm and for grafts with increased SI 4.3 mm (p=0.001). The same was seen in the mean ML thickness of AM grafts (5.9 vs. 5.5 mm, p=0.127) and PL grafts (5.3 vs. 4.9 mm,p=0.171).
The SI of intact grafts and partially ruptured grafts was evaluated. The SI was increased in 60% of patients with a partial rupture of the AM graft and in 14% of patients with an intact AM graft (p= 0.032). In PL grafts, SI was increased in 93% of patients with a partial rupture and in 51% of patients with an intact graft (p=0.005).
Cystic degeneration of the graft was seen with a partial tear of the PL graft in 100% of patients (p=0.027), with a partial tear of the AM graft in 20% of patients, and also with a disrupted AM graft in 20% of patients (p=0.083).
Impingement of the AM graft was seen with a partial tear of the AM graft in 60% of patients (p=0.003) and with increased SI of the PL graft in 80% of patients (p=0.151).
Discussion
The number of ACL DB graft reconstructions has increased in recent years because the procedure results in better rotational stability than traditional SB ACL graft recon-struction with fewer graft failures [9,10,14,16,17]. This was shown in a recent review of 14 randomized controlled trials by Järvelä and Suomalainen [32]. Controversially, an earlier meta-analysis [33] of four randomized controlled trials showed no difference in reported clinical outcome measures.
MRI evaluation of traditional SB ACL graft reconstruction and complications is well established in the literature [18–24], but there are only a moderate number of publications of DB MRI imaging [17, 25–27, 34]. According to Casagranda et al., the complications of SB and DB ACL reconstructions are similar [25].
In our study both grafts were measured in the standard Table 5 Percentages (%) and number of patients with increased SI in
intra-articular parts of AM and PL grafts
Proximal part Middle part Distal part AM grade II 67% (4/6) 100% (6/6) 50% (3/6) AM grade III 100% (5/5) 100% (5/5) 80% (4/5) PL grade II 85% (17/20) 90% (18/20) 50% (10/20) PL grade III 83% (15/18) 100% (18/18) 72% (13/18) SISignal intensity,AManteromedial,PLposterolateral
Fig. 4 MRI of cystic degeneration of the PL graft inside the tibial
Skeletal Radiol (2012) 41:835–842 839
AM graft was 5.8 mm and the thickness of the PL graft was 4.6 mm. The mean ML thickness was 5.9 and 5.1 mm, respectively. The interobserver agreement was excellent at the mean difference of 0.03–0.09 mm (Table3). In a study of the MRI measurements of the two bundles of ACL, the mean AP thickness of the native AM bundle was 5.1 mm, the mean AP thickness of the native PL bundle was 4.4 mm, and the mean ML thickness was 4.2 and 3.7 mm, respectively [35]. In the same study 10 knees were evaluated with both arthroscopy and MRI. Arthro-scopic measurements were larger (AM bundle 6.8 vs.
5.1 mm, PL bundle 5.4 vs. 4.4 mm), but the difference was not statistically significant [35]. Our measurements are somewhat larger than the native ACL MRI measure-ments reflecting the sizing of the grafts [9]. Anatomically, the width of the native ACL in cadavers is 7–12 mm (mean 10 mm) [3].
In our study, 3% of patients had a disrupted AM graft, 6% of patients a disrupted PL graft, and 3% of patients had both grafts disrupted. Previously, Fu et al. reported DB graft failure with both grafts disrupted in 8% of patients with surgical confirmation in a 2-year prospective study [31]. Recently, van Eck et al. described surgically con-firmed patterns of DB reconstruction re-rupture in a cohort of 40 patients presented for revision surgery [36]. The most common pattern (35%) was a mid-substance rupture of the AM graft with a mid-substance rupture of the PL graft, while in 19% of patients the PL graft was intact. In our cohort, more PL graft disruptions were seen but our study lacks the surgical confirmation of the findings.
In MRI evaluation, we based the recognition of graft disruption on the discontinuity of graft fibers. This MRI finding was the most reliable (sensitivity 72% and specificity 100%) in a study by Collins et al. [22] of surgically confirmed graft disruption. In their group, the comprehensive assessment of other previously described MRI primary findings of graft disruption such as graft thickness, graft SI, and graft orientation did not further increase sensitivity.
The most reliable MRI finding of a partial graft tear is the focal thinning of the graft [22]. In our study the graft thickness was measured at a distance of 5 mm from the tibial plateau, but the whole length of the graft was examined for focal thinning. The number of partial ruptures of PL grafts was relatively high in our study (23%) compared to AM bundles (8%).
Increased PD-weighted and T2-weighted SI alone is not a reliable sign of partial rupture [23,28]. In our group, 14%
of intact AM grafts and 51% of intact PL grafts demonstrated increased SI in MRI. The most important reason for the different proportions of increased graft SI seems to be the MRI examination protocol used (Table1)
oblique coronal T1 sequences prescribed from the inter-condylar roof or the Blumensaat line along the course of the AM graft. The PL graft courses obliquely in all these sequences leading to the possibility of volume-averaging the PL graft signal with surrounding intermediate-signal material. A similar phenomenon resulting in increased SI has been described for native PL bundle in a study comparing MRI appearance with corresponding anatomic and histologic sections in cadavers [37].
Saupe et al. [23] found no clinical or functional correlation with increased SI of the graft in PD-weighted and T2-weighted images 4–12 years after SB ACL reconstruction. Controversially, in the first study of DB ACL reconstruction MRI, the increased SI of the graft did correlate with laxity in functional testing [27]. In that study, the SI increase in the graft was evaluated with T2-weighted images using TR 4,200, TE 90 showing 20% of AM grafts and 21% of PL grafts with grade II increased SI 1 year after operation [27]. In their MRI protocol, oblique sagittal and coronal images were obtained for each graft minimizing the effect of volume-averaging [27]. Compared to their results, the AM graft SI was similarly increased in our group (17 vs. 20%), but the PL graft signal was increased more often (61 vs. 21%) [27]. The greater proportion of PL grafts with increased SI in our cohort may be due to volume-averaging artefacts. Unfortunately, 52% of patients in our study underwent MRI prior to the designed 24 months postoperatively (mean time from operation 22 months). This might explain some increased SI caused by graft ligamentization or remodeling as seen in both SB [20,28] and DB reconstructions [25].
Another explanations for increased T1-weighted and T2-weighted SI of native ACL include eosinophilic [37] or mucoid degeneration [38–40] similar to that seen in rotator cuff tendons [41]. ACL cystic degeneration and mucoid degeneration are both due to the mucinous degeneration of the connective tissue of the ligament, and they may also coexist [39, 40]. In native ACL, mucoid degeneration is usually an incidental finding not associated with instability [39]. In our patients, increased graft SI was always seen in both T1- and T2-weighted images, which may be due to mucinous degeneration. For SB ACL grafts, another hypothesis for increased SI on both T1-weighted and T2-weighted images is alterations in the collagen structure of the graft caused by instrumentation or mechanical stress [42].
Association between MRI findings was evaluated. In our group thinning of the graft and a partial rupture were associated with the increased SI of the graft. Cystic degeneration of the graft was associated with a partial rupture or disruption of the graft. Impingement of the AM graft was associated with a partial tear of the AM graft and
840 Skeletal Radiol (2012) 41:835–842
Limitations of our study include the lack of oblique sagittal and coronal sequences along the course of the PL graft resulting in volume-averaging of PL graft SI in orthogonal sequences. Another limitation is the lack of surgical confirmation of the findings of this study. Because of the prospective nature of the study, the follow-up MRI was performed 2 years after operation and for most patients there was no indication for re-arthroscopy.
In summary, this study describes the 2-year postopera-tive MRI findings and the frequencies of complications of DB ACL grafts. AM and PL graft thicknesses were near original graft thickness after operation. The AM graft was partially torn in 8% and disrupted in 3% of patients. The PL graft was partially torn in 23% and disrupted in 6% of patients. Both grafts were disrupted in 3% of patients. For the clinical point of view, the disruption of both grafts is an important finding because it can lead to revision ACL surgery if the patient complains of symptoms of instability in the operated knee as well. Luckily, only 3% of the patients had both grafts disrupted. However, only the following years will reveal if the partial tears of the grafts seen in MRI will lead to total disruption of the grafts and instability symptoms of the operated knees with a need for a revision ACL surgery. We will continue the study follow-up to find out the long-term MRI findings, and the revision rate of the patients needing revision ACL surgery because of graft failure and instability symptoms.
Conflict of interest The authors declare that they have no conflict of interest.
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