The Superficial Medial Collateral Ligament Is the Primary Restraint to External Tibial Rotation Among Medial Knee Structures: A Robotic Biomechanical Study

Abstract

Background:

External rotational stress of the knee leads to several knee problems and persistent pain. Clarifying the role of knee structures in external rotational stability aids in optimizing nonoperative treatment and guiding surgical indications. No previous studies have simultaneously compared the biomechanical contributions of both medial and lateral soft tissue structures to external rotational stress using a robotic system.

Purpose:

To investigate the influence of multiple soft tissue knee structures on stability during external tibial rotation at 0° to 90° of flexion using a robotic testing system.

Study Design:

Descriptive laboratory study.

Methods:

A total of 9 fresh-frozen cadaveric knee specimens and a robotic testing system were used. First, 5 N·m of external tibial rotation was applied to the intact knee at 0°, 15°, 30°, 60°, and 90° of knee flexion. The anterior cruciate ligament, anterolateral capsule, lateral collateral ligament, popliteus tendon (PT), posterior root of the lateral meniscus, superficial medial collateral ligament (sMCL), posterior root of the medial meniscus (MMPR), and posterior cruciate ligament (PCL) were then completely transected in sequence. After each transection, intact knee motion was reproduced for each knee condition, applying 5 N·m of external tibial rotation. By employing the principle of superposition, the resultant force of each structure was determined based on the 6 degrees of freedom force/torque data of each state. Resultant forces were statistically compared using the Kruskal-Wallis test, followed by the post hoc Steel-Dwass test.

Results:

The sMCL exhibited the greatest resultant force across all knee flexion angles from 0° to 90°. Between 30° and 90°, the MMPR and PT generated the highest resultant forces after the sMCL, while the PCL showed the greatest force at 90° after the sMCL, MMPR, and PT. At 60° of knee flexion, the sMCL, MMPR, and PT showed significantly greater resultant forces than the other structures (P < .05).

Conclusion:

Our study demonstrated that the sMCL exhibited the greatest resultant force under external tibial rotation across all knee flexion angles from 0° to 90°.

Clinical Relevance:

This study emphasizes the need to avoid excessive external rotation during the nonoperative or postoperative management of sMCL injuries, as rotational stress may compromise healing and functional recovery of the sMCL.

Keywords

biomechanics sMCL external rotation 6 degrees of freedom (DOF) robotic system fresh-frozen cadaveric specimens

Previous studies have analyzed the anatomy and role of the medial¹⁵ and lateral¹² structures in external rotational stability. Ligaments such as the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), and lateral collateral ligament (LCL) improve knee joint stability by restricting specific joint movements.^16,26,31 In addition to that, nonligamentous soft tissue structures, including the meniscus, also contribute to knee joint stability.^5,27,28,32 External rotational instability of the knee leads to ACL reruptures and persistent chronic pain, thereby hindering the return to sports.^9,17,21,25 Therefore, maintaining external rotational stability is important. While conventional orthotic therapy can control varus and valgus instability, it is less effective in managing internal and external rotational stress.^10,19 Clarifying the role of each knee structure in maintaining external rotational stability will aid in optimizing nonoperative treatment strategies and indications for surgery. Regarding external rotation, numerous biomechanical studies have examined the structures involved in maintaining external rotational stability of the knee joint; these studies have identified the superficial MCL (sMCL) as the primary restraint to external rotation.^{2,3,6,11,16,23,29,33,35}

A previous study demonstrated that both the sMCL and ACL contribute to anteromedial rotatory stability; however, it did not simultaneously investigate multiple soft tissue structures.² The role of various medial structures in rotational stability has also been reported.^14,16 However, these previous studies have either compared only a few structures or examined multiple structures but focused solely on medial knee joint structures. Although previous studies have elucidated the anatomic and biomechanical function of the MCL, few have quantitatively compared its contribution to external rotational stability with other medial and lateral soft tissue structures. They were generally limited to analyzing 1 or 2 structures, such as the popliteus tendon (PT), the ACL, the posterior root of the lateral meniscus (LMPR), and the anterolateral capsule (ALC), at a time because of technical constraints.^22,36 These limitations have made it difficult to comprehensively understand the relative biomechanical contributions of multiple soft tissue structures to external rotational stability.

However, a 6 degrees of freedom (DOF) robotic system can accurately reproduce intact knee motion even after the structure is dissected, allowing biomechanical testing to be performed without the effects of joint instability.^7,8 Thus, the purpose of this study was to quantitatively compare the biomechanical contributions of multiple medial and lateral soft tissue structures to external rotational stability of the knee using a robotic testing system. We hypothesized that the sMCL would provide the greatest resistance to external tibial rotation among all examined soft tissue structures.

Methods

Specimen Preparation

The experimental protocol was approved by the ethics committee of our institution (No. 1-2-68). A total of 9 fresh-frozen human cadaveric knee specimens (5 male and 4 female; mean age at death, 85.2 years [standard deviation (SD) ± 4.7 years]) were included in this study. All cadaveric donations were made after obtaining informed consent provided before death. Before biomechanical testing, each specimen underwent a manual assessment to ensure ligamentous integrity and a full range of motion from complete extension to 130° of flexion; specimens exhibiting instability or motion limitations were excluded. Specimens were thawed for 24 hours at ambient temperature before testing and kept moist throughout the experiment to maintain structural quality. Each femur and tibia was transected approximately 15 cm above and below the joint line, respectively, and the fibula was sectioned 5 cm distal to the proximal tibiofibular joint. Soft tissue structures, including the quadriceps muscle, hamstring muscle, and patella, were removed, whereas the knee ligaments, joint capsule, and menisci were preserved. Both the femur and tibia were embedded in cylindrical molds with acrylic resin (Ostron II; GC) and firmly secured using aluminum clamps. The fibula was likewise fixed in its native anatomic position with resin. Finally, the prepared specimens were mounted onto a 6-DOF robotic testing system (FRS-2010; Technology Services) using the cylindrical attachments (Figure 1). All sensors were calibrated according to the manufacturer's guidelines before testing. Specimen alignment and initial joint orientation were adjusted manually to match the neutral anatomic position based on visual and tactile landmarks before embedding.

Figure 1.

A robotic testing system was used to assess a right knee. A manipulator equipped with a universal force/torque sensor was mounted onto the end effector. The tibia was attached to the end effector, while the femur was secured to the base of the device using metal clamps.

Testing Apparatus

A 6-DOF robotic testing system (FRS-2010) equipped with a custom-designed manipulator and a universal force/torque sensor (DELTA IP65, SI-660-60; ATI Industrial Automation) was utilized (Figure 1). This robotic setup enabled precise, reproducible loading, regardless of joint instability, even after sequential dissection based on the in vitro joint coordinate system described by Grood and Suntay.¹³ Real-time control of displacement and the application of force/torque along each axis were achieved using a custom LabVIEW program (Version 12.0.1; National Instruments).

Testing Protocol for Intact State

First, the 0° of flexion position was established by applying a 0.5-N·m extension moment to the knee. Subsequently, passive flexion-extension movements were performed from a hyperextended position to 120° of knee flexion, with a constant 5-N·m extension moment at a rate of 0.5 deg/s.

This preconditioning cycle was repeated 3 times for each specimen. After preconditioning, an external tibial rotation test was conducted by applying a 5-N·m external rotational torque at 0°, 15°, 30°, 60°, and 90° of knee flexion. Throughout these procedures, 6-DOF knee motion and corresponding force/torque data were recorded under intact conditions. The femoral and tibial coordinate systems were described based on the joint coordinate system of Grood and Suntay¹³ in which external tibial rotation was defined as rotation about the floating axis perpendicular to the plane formed by the flexion-extension and varus-valgus axes.

Transection of Structures

Sequential transection of the ACL, ALC, LCL, PT, LMPR, sMCL, posterior root of the medial meniscus (MMPR), and PCL was performed in the specified order. The transection order was determined based on the anatomic configuration, as this specific order allowed for the most efficient and accurate dissection.^7,8

The ACL, LCL, sMCL, and PCL were transversely cut at the midsubstance of the ligament. The ALC was divided immediately superior to the lateral meniscus; no distinct anterolateral ligament was identified in the specimens analyzed.³⁴ The PT was incised just distal to its femoral attachment. The LMPR and MMPR were transected approximately 5 mm from their respective posterior root. Pilot studies have compared complete meniscectomy with isolated posterior root transection and have demonstrated that posterior root tears alone effectively abolished meniscal force contributions under anterior tibial loading, in line with previous findings under axial loading conditions.¹ Thus, considering specimen damage from the cutting sequences of this experimental protocol, we selected the posterior root tear as a representative state when the meniscus's function was completely lost.

Testing Protocol for Transected State

The contribution of each structure was determined by employing the principle of superposition^7,8 for the 6-DOF force/torque data obtained at each stage. Initially, 5-N·m external tibial rotation was applied to the intact knee, and the resulting motion and force/torque data were recorded. After each subsequent transection, the same motion was reapplied, and the corresponding force/torque measurements were collected to calculate the resultant force for each structure (Table 1).

Table 1

Experimental Protocol to Determine Force of Each Structure ^a

Stage	Loading	Data Acquired	Resultant Force Calculated
(Intact)	5-N·m external tibial rotation	Force 1	—
ACL cut	Repeat	Force 2	Resultant force of ACL (difference between force 1 and force 2)
ALC divide	Repeat	Force 3	Resultant force of ALC (difference between force 2 and force 3)
LCL cut	Repeat	Force 4	Resultant force of LCL (difference between force 3 and force 4)
PT transection	Repeat	Force 5	Resultant force of PT (difference between force 4 and force 5)
LMPR tear	Repeat	Force 6	Resultant force of LMPR (difference between force 5 and force 6)
sMCL cut	Repeat	Force 7	Resultant force of sMCL (difference between force 6 and force 7)
MMPR tear	Repeat	Force 8	Resultant force of MMPR (difference between force 7 and force 8)
PCL cut	Repeat	Force 9	Resultant force of PCL (difference between force 8 and force 9)

ACL, anterior cruciate ligament; ALC, anterolateral capsule; LCL, lateral collateral ligament; LMPR, posterior root of the lateral meniscus; MMPR, posterior root of the medial meniscus; PCL, posterior cruciate ligament; PT, popliteus tendon; sMCL, superficial medial collateral ligament.

Statistical Analysis

Statistical analysis was performed using EZR (Saitama Medical Center, Jichi Medical University), a graphical user interface for R (R Foundation for Statistical Computing).¹⁸ At each flexion angle (0°, 15°, 30°, 60°, and 90°), the resultant forces of the 8 knee joint structures were compared using the Kruskal-Wallis test. If a significant difference was observed, the post hoc Steel-Dwass test was performed for multiple comparisons. A P value <.05 was considered statistically significant.

Results

The resultant forces of the structures in the intact knee in response to external tibial rotation are shown in Figure 2. At each knee flexion angle, the resultant force of the sMCL was greater than that of the other structures. Between 30° and 90° of knee flexion, the resultant force of the MMPR and PT was larger than that of all other structures, excluding the sMCL. At 90° of flexion, the resultant force of the PCL was larger than that of all other structures, excluding the sMCL, MMPR, and PT.

Figure 2.

Resultant forces of knee structures in response to external tibial rotation. Statistical comparisons were performed using the Kruskal-Wallis test with the post hoc Steel-Dwass test. Structures labeled with a letter (a) are significantly different from unlabeled structures. Brackets indicate significant differences at the same knee flexion angle. Daggers (†) indicate structures that showed statistically significant differences (P < .05; Steel-Dwass test) compared with the anterior cruciate ligament (ACL), anterolateral capsule (ALC), lateral collateral ligament (LCL), posterior root of the lateral meniscus (LMPR), and posterior cruciate ligament (PCL). MMPR, posterior root of the medial meniscus; PT, popliteus tendon; sMCL, superficial medial collateral ligament.

Discussion

The major findings of our research are that the sMCL exhibited the greatest resultant force under external tibial rotation at all knee flexion angles from 0° to 90°. Between 30° and 90°, the MMPR and PT generated the highest resultant forces after the sMCL. Excluding the sMCL, MMPR, and PT, the PCL's resultant force was also large at 90°. At 60°, the sMCL, MMPR, and PT showed significantly greater resultant forces than the other structures (P < .05). The sMCL consistently contributed significantly at all angles, playing a crucial role in knee external rotation.

It is possible that the reason why the sMCL plays a significant role in external rotation can be explained by its anatomic fiber orientation. The sMCL originates slightly posterior to the medial epicondyle of the femur and inserts distally on the anteromedial aspect of the tibia, approximately 6 cm below the joint line. This oblique alignment, running from the posteromedial femur to the anteromedial tibia, becomes taut during external tibial rotation, generating a restraining force against excessive external rotation.²⁰ Additionally, the sMCL shows increased tension during external rotation throughout the full range of knee flexion (0°-90°).¹¹ Our findings demonstrated that the sMCL plays a significant role in rotational stability throughout the full range of knee flexion, which is consistent with previous in vitro results showing that anatomic reconstruction of the sMCL restores valgus and external rotational stability in cadaveric models.⁴

The management of sMCL injuries depends on injury severity and involves either nonoperative or surgical treatment. Nonoperative therapy focuses on early rehabilitation and is generally associated with good clinical outcomes.⁴ For severe sMCL injuries that lead to bad clinical outcomes with nonoperative management, surgical treatment is indicated.²⁴

The sMCL has a greater force contribution than the MMPR in external rotation movements.¹⁴ Although previous reports have suggested that the popliteus muscle is an important stabilizer for external rotation, the number of biomechanical studies directly evaluating its role remains limited.²² To our knowledge, no previous studies have directly compared the relative contributions of the sMCL and popliteus muscle/tendon in external rotation. In this study, we found that both are important for external rotation movements. However, the sMCL had a greater force contribution. Furthermore, this study highlights the significant role of the PT in external rotation. Posterolateral injuries of the knee joint, including the LCL and the PT, have been increasing because of the recent rise in sports injuries. These injuries are often associated with ACL and PCL damage and may go undiagnosed initially. If left untreated, they can lead to chronic pain and knee instability.³⁰ The role of the PT in external rotation should be further investigated in clinical contexts, as this study identified a substantial biomechanical contribution at higher flexion angles.

Limitations

This study has several limitations. First, as the experiment was conducted using specimens from older donors, there is a possibility that poor bone quality and degenerative changes in the tendons could have influenced the results. Although no apparent degenerative alterations were identified in the specimens used, such factors might still introduce bias. Second, because testing was performed in an in vitro setting, the effects of muscle forces and soft structures such as the anterior capsule and extensor mechanisms, which had been removed during specimen preparation, were not accounted for. Third, only specific flexion angles were evaluated, and forces at other flexion positions were not assessed. Fourth, the study focused solely on evaluating flexion/extension forces in response to external tibial rotation; additional testing involving rotational stability would provide more comprehensive and clinically relevant insights. Fifth, this study lacked an a priori power analysis to determine the required sample size. Future studies should incorporate power calculations to ensure adequate statistical power. Finally, the sequential order of structure transection was not randomized, which could potentially introduce bias. However, the order was based on anatomic considerations to ensure accurate dissection. Furthermore, the robotic testing protocol employed in this study was based on the principle of superposition, which allowed for the reproduction of identical joint kinematics before and after each transection. This approach theoretically minimizes the influence of the transection sequence, as each structure's biomechanical contribution is evaluated under matched motion and loading conditions.²⁹ Despite these limitations, the present biomechanical data regarding the distribution of forces among the knee structures contribute valuable information toward a better understanding of human knee joint biomechanics.

Conclusion

Our study demonstrated that the sMCL exhibited the greatest resultant force under external tibial rotation at all knee flexion angles from 0° to 90°. Between 30° and 90°, the MMPR and PT generated the highest resultant forces after the sMCL. The sMCL is a primary stabilizer against external rotation, and its involvement should be considered in both nonoperative treatment strategies and indications for surgery.

Footnotes

Acknowledgements

The authors thank those who donated their remains to this study.

Final revision submitted September 11, 2025; accepted October 6, 2025.

One or more of the authors has declared the following potential conflict of interest or source of funding: This work was partially funded by a Ministry of Education, Culture, Sports, Science and Technology/Japan Society for the Promotion of Science KAKENHI grant (JP 20K18035 to K.S.). AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

Ethical approval for this study was obtained from the ethics committee of Sapporo Medical University (No. 1-2-68).

ORCID iDs

Rikiya Itagaki

Hidenori Otsubo

Tomoaki Kamiya

Daisuke Suzuki

Satoshi Yamakawa

Shogo Nabeki

Tomoyuki Suzuki

Makoto Emori

Atsushi Teramoto

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