Introduction

Radiographic identification of the anatomic attachment sites of knee ligament structures allows surgeons to obtain an accurate diagnosis and treatment of knee injuries. The previous studies have provided quantitative and radiographic data for other ligaments [1–6], which has aided surgeons in intraoperative tunnel placement [7, 8] and postoperative assessment of anatomic knee ligament reconstructions [7–10]. The qualitative and quantitative anatomic descriptions of the anterior and posterior ligament complexes of the PTFJ are currently becoming better understood due to recent focus on this topic [11, 12]. However, quantitative guidelines for radiographic identification of the anterior and posterior ligament bundles of the PTFJ are still lacking in the current literature.

Due to the complex anatomy of the posterolateral corner of the knee [13, 14], proper identification of tunnel placement is critical to properly diagnose and treat associated injuries to this area. Therefore, the purpose of this study was to provide reproducible, quantitative descriptions of radiographic landmarks identifying the anterior and posterior ligament complexes of the proximal tibiofibular joint to guide intraoperative tunnel placement and postoperative assessment of an anatomic proximal tibiofibular joint reconstruction.

Materials and methods

Specimen preparation

Ten non-paired (n = 10), fresh-frozen human cadaveric knees [median age 57 years (range 47–66 years); 8 male, 2 female] with no history of prior injury, anatomic abnormalities, ligament instability, osteoarthritis, or disease were utilized for the study. The cadaveric specimens utilized in this study were donated to a tissue bank for medical research and then purchased by our institution. Specimens were stored at −20 °C and thawed for 24 h prior to dissection. The femur was disarticulated and all soft tissues (except for the interosseous membrane) were removed distal to the tibiofemoral joint line. Screws were placed through the distal ends of the tibia and fibula of each specimen, with the interosseous membrane intact to ensure native anatomic positioning was obtained, and then potted in polymethyl methacrylate (PMMA) (Fricke Dental, Streamwood, Illinois). The individual bundles of both the anterior and posterior ligament complexes were carefully isolated. The anterior complex was always analyzed before the posterior complex. The tibial side was always analyzed before the fibular side, starting with the most superior bundle on the tibial side, followed by the next bundle inferiorly until the entire ligament complex was analyzed. As each bundle was sectioned from its tibial attachment, a radiopaque marker was placed in the center of its footprint, flush against the bone. This process was repeated on the fibular side.

Data collection

Using a mobile C-arm, standardized, anteroposterior (AP), and lateral radiographs were obtained separately for both the anterior (Fig. 1) and posterior (Fig. 2) ligament complexes of each specimen. The femur of the specimens utilized in this study had been disarticulated, and therefore, the femoral condyles were not available for reference during radiographs. To obtain an AP radiograph, the anterior and posterior margins of the medial tibial plateau were superimposed. To obtain a straight lateral radiograph, both medial and lateral plateaus were overlapped, the tibial tubercle was visualized, and a portion of the proximal head of the fibula (about 1/3 of the fibular head) was superimposed over the tibia, so that an outline of the fibular shaft could be seen posterior to the tibia. A 25.4-mm metal sphere scaling marker was placed at the same height and depth as the proximal tibiofibular joint for each radiograph and used later for magnification/measurement calibration with the PACS OrthoCase Imaging software (Merge Healthcare, Chicago, IL). Measurements obtained with the imaging software were reported to the nearest 0.1 mm.

Fig. 1
figure 1

a Anteroposterior and b lateral fluoroscopic images of a left knee demonstrating the anterior complex of the proximal tibiofibular joint. Of note, four separate bundles were identified in the majority of the specimens. The apex of the fibula was identified as an osseous reference

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Fig. 2
figure 2

a Anteroposterior and b lateral fluoroscopic images of a left knee demonstrating the posterior complex of the proximal tibiofibular joint. Of note, two separate bundles were identified in the majority of the specimens. The apex of the fibula was identified as an osseous reference

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For both AP and lateral radiographs, the proximodistal position of the center of each ligament bundle of the PTFJ was measured from the tibial plateau for tibial insertions, and from the apex of the fibular head for the fibular insertions. This was done by drawing a straight reference line along the entire tibial plateau for AP views and along the medial tibial plateau for lateral views. A line parallel to the previous reference line was then drawn at the apex of the fibular head for both AP and lateral views. Perpendicular line measurements were made from each respective parallel reference line to obtain the proximodistal distance to each ligament’s tibial and fibular attachment site (Figs. 3, 4). The width of the tibial plateau was measured on AP views as a reference for the size of each specimen and to determine if a correlation existed between the size of the specimen and the location of the ligaments. Two observers, both fellowship-trained orthopaedic surgeons (JC & GM), performed an independent round of measurements on each specimen to obtain inter-class measurement reliability. Institutional review board approval was not required, because the use of cadaveric specimens is exempt at Steadman Philippon Research Institute.

Fig. 3
figure 3

a Anteroposterior and b lateral schematic images of a left knee demonstrating the radiographic measurements of the posterior complex of the proximal tibiofibular joint. The tibial plateau surface line and the apex of the fibula were identified as a reference for the tibial and fibular ligaments, respectively. The blue markings represent the center of ligament attachments on the tibia and the red markings represent the center of ligament attachments on the fibular head

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Fig. 4
figure 4

a Anterior and b posterior ligamentous complex of the proximal tibiofibular joint. ALL anterolateral ligament, G Gerdy’s tubercle, TT tibial tubercle, FCL fibular collateral ligament, PCL posterior cruciate ligament. After identification of the individual bundles, radiopaque marker was placed in the center of its footprint, flush against the bone, to be identified for subsequent measurements

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Statistical analysis

Summary statistics were used to characterize radiographically measured anatomic distances. Spearman correlations were calculated to test for association between measurements. Interrater measurement reliability was assessed using the random-effects, single measures, absolute agreement definition of the intra-class correlation coefficient (ICC), reported with non-parametric 95% bootstrap confidence intervals. All analyses and figures were produced using the statistical programming language R [15]. Note that ligament span was defined by calculating the difference between the most proximal and most distal ligament bundle of each respective complex and then averaged across all ten specimens.

Results

Interrater agreement across all specimens, for both AP and lateral views, and for both the fibular and tibial sides was excellent. The lowest agreement was for lateral view of fibular measurements [ICC = 0.993; 95% CI (0.977, 0.997)]. Detailed radiographic measurements for the individual separately identified bundles of the anterior and posterior ligament bundles of the proximal tibiofibular joint can be seen in Tables 1 and 2, respectively.

Table 1 Distance (mm) from bony landmarks to the center of the individual bundles of the anterior complex
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Table 2 Distance (mm) from bony landmarks to the center of the individual bundles of the posterior complex
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Anterior complex

The anterior complex spanned a proximodistal distance of 12.8 ± 3.9 mm on the tibia for AP radiographs and 13.7 ± 3.2 mm for lateral radiographs. The anterior complex spanned a proximodistal distance of 11.6 ± 6.8 mm on the fibula for AP radiographs and 14.6 ± 7.1 mm for lateral radiographs.

Posterior complex

The posterior complex spanned a proximodistal distance of 11.7 ± 8.4 mm on the tibia for AP radiographs and 12.9 ± 9.1 mm for lateral radiographs. The posterior complex spanned a proximodistal distance of 11.8 ± 7.9 mm on the fibula for AP radiographs and 10.9 ± 7.5 mm for lateral radiographs.

Tibial plateau width

The average width of the tibial plateau across all ten specimens was 73.7 ± 5.3 mm. There was no significant correlation between the tibial plateau width and span of the anterior or posterior ligament complexes on the tibia or fibula for either AP or lateral views (n.s.).

Discussion

The most important finding of this study was that quantitative radiographic assessment of the anatomic attachment sites of the anterior and posterior complexes of the proximal tibiofibular joint on standard radiographs could be consistently performed. Due to the thin nature of each individual bundle of the anterior and posterior ligaments and aiming towards a more clinically relevant measure, the span of the anterior and posterior complex was also assessed. This will aid surgeons in choosing tunnel placement for an anatomic reconstruction and provides guidelines to check this tunnel positioning intraoperatively with fluoroscopy.

Dislocation of the proximal tibiofibular joint (PTFJ) is a complex injury that often results in disruption of the posterior ligament complex [16–20]. Conservative treatment with closed reduction and immobilization is often recommended [18, 19, 21]; however, in cases of persistent joint instability following conservative management, surgical treatment is indicated [18, 19, 21–24]. Different surgical techniques have been proposed which include PTFJ arthrodesis [25, 26], fibular head resection [27, 28], re-routing of the biceps femoris tendon [29–31], closed-wedge high tibial osteotomy [32], and PTFJ ligament reconstruction [24, 33, 34]. Non-anatomic surgical treatments such as arthrodesis and fibular head resection are associated with complications such as chronic knee instability and chronic ankle pain [18, 21, 26, 27]; hence, anatomic reconstructions have been advocated [24, 33, 35–38]. Due to the reported improved biomechanical function and patient outcomes following anatomic reconstruction of torn knee ligaments [36, 38–40], acute traumatic or chronic PTFJ instability should be treated similarly [24].

The body of literature describing the anatomy of the proximal tibiofibular joint ligaments is scant, and to the authors’ knowledge, no radiographic assessment of the PTFJ ligaments has previously been performed. Furthermore, there is still no consensus in the literature on the number of bundles in both the anterior and the posterior complexes; however, due to the relatively small size of the individual bundles, treating the complex as a whole would seem most clinically relevant. The current study provides quantitative radiographic data for both the individual ligament bundles and the entire span of the anterior and posterior complexes. Intraoperative use of radiographs can guide surgeons in placing anatomic-based reconstruction graft tunnels, especially in injuries where the tissue cannot be easily identified.

It is acknowledged that there are some limitations to the present study. First, the relatively small number of specimens (n = 10) may underestimate the true anatomical variability of the PTFJ. In addition, the specimens were from donors who were older than the usual age range of patients who undergo a proximal tibiofibular reconstruction (however, the soft-tissue attachment sites and osseous landmarks of interest do not vary with age and the specimens had no evidence of the previous injury or large osteophyte formation).

Conclusion

The attachment locations of the proximal tibiofibular anterior and posterior complexes could be quantitatively correlated to reliable osseous landmarks and radiographic lines. This information will allow for consistent radiographic assessments of proper tunnel placement both intraoperatively and postoperatively during anatomic reconstructions of the proximal tibiofibular joint.