Radiological anatomy and injuries of the condylar strap and Kaplan fibers associated with ACL pathology
How to cite this article: Chandra AA, Chandra A, Murray T, Azam M. Radiological anatomy and injuries of the condylar strap and Kaplan fibers associated with ACL pathology. Indian J Musculoskelet Radiol 2023;5:25-30.
The study aimed to identify the Kaplan fiber (KF) complex, focusing on the condylar strap (CS) which are fiber straps on the lateral aspect of the knee related to the iliotibital complex, on magnetic resonance imaging (MRI) of knees performed for acute anterior cruciate ligament (ACL) injury, measure the distances of insertion from the lateral joint line, and identify injuries to these structures.
Material and Methods:
Retrospective analysis of 134 MRIs of knees with acute ACL tear was performed. The proximal and distal KFs and the CS were identified on at least one plane on MRI. The distance of their femoral attachment from the lateral joint line and injury to these structures with concomitant knee injuries was documented. Data analysis was performed using the SPSS software.
KFs were identified in 97.2% and CS in 72% of the MRI knees. The mean distance of the proximal KF from the lateral joint line was 59.1 mm, while that of the distal KF was 42.4 mm. The CS attached 35.6 mm proximal to the lateral joint line. KF complex injury was present in 34% of cases with ACL tear. The MRI finding of meniscal tear and bone marrow edema in a classic pivot shift impaction injury pattern had a significant positive correlation with KF injury.
This radiological study confirms the existence of CS on MRI. There is a positive association of KF injury with meniscal tears and bone marrow edema in patients with acute ACL tears. Future biomechanical studies focusing on CS are needed to identify its contribution to anterolateral stability.
The anterior cruciate ligament (ACL) is one of the most frequently torn ligaments of the knee. Persistence of instability after ACL surgery has been attributed, in some instances, to instability in the anterolateral structures of the knee, including the anterolateral ligament (ALL), iliotibial band, capsular fibers, and the condylar strap (CS). Anatomical studies have been performed to further characterize these anterolateral structures of the knee. To control persistent anterolateral rotatory instability of the knee, different types of tenodesis have been described. It has been postulated that persistent instability could be due to non-anatomic reconstruction of the anterolateral structures.
There have been studies investigating the role and anatomy of Kaplan fibers (KFs) but few anatomical studies describe the CS (a structure between the femur and the lateral gastrocnemius (LG) on one side and the deep surface of the ITB on the other,) a third stabilizer between the ilio-tibial tract and femur. This particular structure has not been studied in any prior imaging-based published research.
The purpose of this study was to identify the KF complex, focusing on the CS on routine magnetic resonance imaging (MRI) of the knee, to measure the distances of the insertion from the lateral joint line, and identify injuries to these structures associated with ACL injury.
MATERIAL AND METHODS
A retrospective study was performed on 150 MRIs of the knee with acute ACL injury after taking approval from the institutional review board.
A total of 150 MRIs of the knee with acute ACL injury were retrospectively reviewed. Out of these 150 MRIs, nine patients had multi-ligamentous injuries, two had a history of prior ACL repair with reinjury, and five had displaced proximal tibial/fibular or distal femoral fractures. These cases were excluded from the final cohort.
All the scans were performed on a 1.5-tesla scanner, using a 15-channel dedicated knee coil. Routine MRI protocol included axial, coronal, and sagittal proton density images (TR 3160 ms, TE 21 ms, field of view 150 ms), axial proton density fat-saturated images (TR 3160 ms, TE 21 ms, field of view 150 ms), T2 turbo spin echo fat-saturated coronal images, T1 coronal images (TR 500 ms, TE 9 ms, field of view 140 ms), and sagittal gradient echo images. The slice thickness was 3 mm with an inter-slice gap of 0.5 mm.
The proximal bundle of the Kaplan fibers courses in a nearly transverse orientation from the ITB to the lateral femur. The distal bundle has a near oblique course, coursing from proximal lateral to distal medial, attaching on a lateral femoral ridge. The lateral geniculate artery courses are inferior to this bundle. The CS is another fiber tract identified as attaching distal to the lateral geniculate artery on the LG tubercle of the femur, with its fibers blending with the LG. On coronal images, these fibers course anteriorly and inferiorly along the ilio-tibial tract, correlating with the “retrograde fiber tract” described by Lobenhoffer et al.  [Figure 1].
All scans were independently reviewed by a fellowship-trained musculoskeletal radiologist. The proximal and distal KFs and the CS were assessed on coronal, sagittal, and axial non-fat saturated PD images to define their radiological anatomy. The axial plane was found to be most suitable for the identification of these fibers. The sagittal and coronal images helped assess the fiber orientation. Identification of the superior lateral geniculate artery coursing just below the insertion of the distal KF added to the diagnostic confidence, and this was best assessed on sagittal images [Figure 2].
To assess the distance of insertion of these fibers from the lateral joint line, a straight line was drawn along the lateral tibiofemoral joint cartilage on coronal images. The site of insertion of these fibers was measured on the coronal plane using multiple series viewing and cross-reference lines [Figure 2]. Fat-saturated proton density and T2 images were then used to identify tearing or edema around the fibers.
Normal fibers (hypointense in signal) were graded as 1, and edema around the fibers with maintained continuity was graded as 2 [Figure 3]. Differentiating partial from complete tears is not always possible on MRI, as also established by Berthold et al. Thus, partial or complete injuries were collectively identified and graded as 3 [Figure 4]. Coexisting injuries within the knee joint were documented, including the pattern of bone marrow edema, if present. Before data analysis, all cases with multi ligamentous injury of the knee or fracture around the knee joint were excluded to prevent confounding, giving a final sample size of 134 patients.
Existing literature was extensively reviewed to identify the parameters used to assess injury to these fibers since their identification is at present limited to anatomical studies. Published literature on the radiological identification of KFs and anatomical identification of the CS served as the reference standard.
For data analysis, SPSS version 26 software (SPSS Inc, Chicago, Illinois, USA) was used. Continuous variables (demographic data) are presented as mean ± SD and categorical variables as percentages and/or numbers. For analysis of the association between KF/CS injury, meniscal injury, and bone marrow edema (categorical variables), Chi-square or Fisher exact tests were employed. P < 0.05 was considered statistically significant throughout the analysis.
The age range of the patient population was 16–42 years. Out of the 134 knees, 78.4% (n = 105) were males. There were a total of 70.1% (n = 94) of right knees and 29.9% (n = 40) of left knees. All MRIs were performed within 3 months of the initial injury. Contact sports accounted for 46% of total cases while a road traffic accident was the initial injury in 42% of cases.
The KFs were identified in 97.2% of the cases in at least one plane. The CS was identified in 72% of the MRI knees. There was one case where only the CS was identified and two cases where only one out of the two KF bundles was identified. The distal bundle of the KF was universally related to the lateral geniculate artery which traversed inferior to the bundle, coursing posterior to anterior.
The mean distance of the proximal KF from the lateral joint line was 59.1 ± 6.9 mm, range 46–75 mm, while the distance from the distal KF to the lateral joint line was 42.4 mm ± 5.6 mm, range 31–60 mm. The CS attached 35.6 ± 4.8 mm proximal to the lateral joint line, with a range of 26– 45 mm, with its attachment corresponding to the LG tubercle.
KF and CS injury was present in 34% of cases with ACL tear, 19% of which occurred in cases with a history of contact sport. The MRI finding of the meniscal tear had a statistically significant positive correlation with KF injury (P = 0.003). In addition, a statistically significant positive correlation was found between a classic pivot shift impaction injury pattern bone marrow edema and KF injury (P < 0.001). There was no incidence of isolated CS injury.
Numerous historical studies have detailed the complex anatomy of the anterolateral aspect of the knee. Kaplan described the eponymous KFs in 1958, and since then many anatomical and few radiological studies focusing on the identification of these fibers and their pathology have been published. Lobenhoffer et al. in 1987 documented the existence of three distinct fiber tracts arising from the iliotibial tract and inserted on the distal femur – the proximal and distal KFs, and a third bundle called the retrograde fiber tract. Presumably, this retrograde fiber tract is the same structure that Muller had referred to as the “lig. Femoro-Tibiale laterale anterius” and Terry et al. had included in his description of the “capsulo-osseous layer.” Godin et al. also described this third strap as a distinct capsuloosseous layer intimately related to the lateral knee capsule in close proximity to the LG tubercle. This retrograde fiber tract was recently again identified in an anatomical study and labeled as CS. This terminology has been used in the present study and is the first study in the literature systematically describing this third fiber tract on imaging.
A radiological study assessing injury to the KF complex by Berthold et al. found an average distance of the proximal fibers complex from the lateral joint line to be 57.6 mm and that of the distal fiber complex to be 44.9 mm on MRI. In another study, correlating MRI with anatomical dissection, Berthold et al. identified the fibers in 100% of cases with Distance from the lateral joint line to proximal KFs 63.5 ± 7.6 mm and the distal KFs 45.3 ± 3.7 mm. They reiterated the close topography of these fibers to the superior lateral genicular artery on the anatomical dissection. Batty et al. performed MRI on 50 patients and identified the mean distance of the distal fibers to be 50.1 mm from the lateral joint line. The fiber was consistently identified as just superior to the lateral geniculate artery on sagittal images in all cases. Sayac et al. recently performed anatomical dissection in 21 non-paired cadaveric knees and identified the proximal and distal fibers lying approximately 49.20 ± 7.38 and 27.54 ± 7.69 mm from the lateral epicondyle. Godin et al. measured the proximal and distal bundle insertions to the 53.6 mm and 31.4 mm proximal to the lateral epicondyle, respectively, inserting on a distinct ridge. These measurements are within the range of the measurements in our study, inferring that there is no anatomical variation in the Indian population. This is also supported by the cadaveric study done by Raghavan et al. who noted a consistent supracondylar attachment of the KFs, either as a single or double limb, 30–40 mm from the lateral femoral epicondyle without any statistically significant difference between Caucasian and Asian groups.
Jakob et al. first described the contribution of KF to knee pivot shift injury. There is contradictory data in the literature with regard to the significance of injury to the KFs in the clinical setting. In the present study, a statistically significant positive correlation was noted between KF/CS injury and meniscal injury, and the presence of bone marrow edema in a pivot shift impaction pattern. Fiber orientation identified in various anatomical studies suggests that the KFs play an important role in the rotatory knee stability, and this is further supported by biomechanical studies.[14,15] Smith et al. emphasized that the distal ilio-tibial band is more important than the ALL for control of pivot shift. Berthold et al. found that 53–56% of cases initially diagnosed as isolated ACL tears had concomitant KF injury, which can contribute to persistent anterolateral rotatory instability. Marom et al. found substantial reliability for the identification of proximal fibers and moderate reliability for the distal fibers. The associated injury was seen in the majority of their cases with acute ACL tear. In the present study, 34% of all cases had concomitant KF injury. The lower incidence of KF injury with acute ACL injury may be due to a large number of cases in this study being secondary to road traffic accidents (42%). Clinically relevant anterolateral rotatory instability persists in 15% of cases following ACL reconstruction; the International ALC Consensus Group Meeting has further acknowledged this by stating the need for future studies to establish indications for anterolateral reinforcement/reconstruction in patients undergoing primary ACL repair.
Godin et al. and Wytrykowski et al. performed biomechanical studies by externally loading the ITB in cadaveric studies. They found that both KFs are strong enough to affect the rotatory stability of the knee, with the distal fiber being stiffer than the proximal fiber. This difference is likely due to the geometric orientation of the fibers. A cadaveric biomechanical study by Smith et al. found that ACL deficient knees return to a native biomechanical state following anterolateral reconstruction with ACL repair, and this minimally invasive anterolateral reconstruction was equivalent to the more extensive modified Lemaire reconstruction. At 0°, the distal ITB plays a more important role than the ALL in controlling anterolateral rotatory movement. Considering these biomechanical, anatomical, and clinical studies, Gali et al. described a novel procedure for anatomical reconstruction of the KF by re-tensioning the ilio-tibial band. This was based on preceding studies which found a significant association between ITB injuries and grade of pivot shift, and not between ACL injury and grade of pivot shift, thus emphasizing the role of ITB and its fibrous attachments in preventing rotatory instability.
Watanabe et al. found KF injury in 23.5% of cases but stated that concomitant fiber injury did not affect the pivot shift phenomena in acute ACL injuries. Contradictory to our study, Lynch et al. did not find any significant association between KF injury and concomitantly observed injuries in conjunction with ACL tear (including meniscal injuries and bone marrow edema), concluding that identifying KF injury does not help surgeons in distinguishing higher grades of rotatory instability. Devitt et al. found no difference in clinical or functional outcomes in patients undergoing ACL reconstruction with or without evidence of KF injury on imaging. Devitt et al. another study found that KF injuries were uncommon, and the rate of Grade III pivot shift was low, suggesting that the role of KF in controlling anterolateral rotatory laxity in acute ACL injury in the clinical setting may be less evident as compared with the biomechanical setting. This may in part be due to a delay between MRI and clinical examination, which could affect the grade of pivot shift injury. In the same study by Devitt et al., the authors have mentioned an association between KF injury and lateral meniscal injury.
The present study has certain limitations. As there was a single reader, inter-reader reliability was not assessed. Cadaveric study by Godin et al. found the CS universally. The lower rate of identification in our study may in part be due to the studies being performed on a 1.5T magnet and anatomical variations in the population. This is also supported by the study of Lobenhoffer et al. who did not find the retrograde fiber tract in all specimens and speculated this to either be due to biological or congenital variation.
This radiological study confirms the existence of the CS by identifying it on MRI, which has previously been described only in cadaveric studies. We reiterate the positive association of KF injury with meniscal tears and bone marrow edema in patients with acute ACL tears. The radiological anatomy of KF and CS is described with femoral attachments found to be in the range of the previous cadaveric studies. All cases with KF injury had concomitant CS injury, but there was no case with isolated CS injury. Identifying injuries to these bundles may help the surgeon in identifying patients at risk for persistent instability following ACL reconstruction with or without ALL reconstruction. Future biomechanical studies focusing on CS are needed to identify its contribution to anterolateral stability.
Declaration of patient consent
Patient’s consent not required as patient’s identity is not disclosed or compromised.
Conflicts of interest
There are no conflicts of interest.
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