Spontaneously reduced extrusion of lateral meniscus following lateral meniscal repair and anterior cruciate ligament reconstruction: a case report

Abstract

Introduction and importance:

Anterior cruciate ligament (ACL) injuries are frequently accompanied by meniscal tears and extrusion. While meniscal repair is commonly performed concurrently with ACL reconstruction (ACLR), persistent postoperative extrusion can occur, and its clinical significance remains uncertain. The spontaneous reduction of such extrusion is a potentially important but underreported outcome.

Case presentation:

We report a case of multi-ligament knee injury involving a complete ACL rupture, medial collateral ligament (MCL) tear, and a lateral meniscal tear with significant extrusion. The patient underwent arthroscopic double-bundle ACLR, inside-out repair of the lateral meniscus, and suture anchor repair of the MCL. A 3-month postoperative MRI confirmed persistent lateral meniscal extrusion. However, subsequent MRI at 6 months showed evident reduction, with near-complete anatomical restoration achieved by 12 months.

Clinical discussion:

This case demonstrated that early residual extrusion following meniscal repair may not necessarily indicate surgical failure. The observed delayed, spontaneous reduction suggests that restored knee stability (via ligament reconstruction) and optimal load distribution may allow the healed meniscus to progressively regain functional integrity. This process may be facilitated by the gradual restoration of circumferential collagen fibers and hoop stresses, rather than representing an immediate postoperative outcome.

Conclusion:

Early postoperative meniscal extrusion may not definitively indicate repair failure and could be monitored over time. Restored knee stability and optimal load distribution may facilitate spontaneous reduction, likely through the gradual restoration of circumferential fiber integrity and hoop stress.

Background

Menisci are fibrocartilaginous structures located within the tibiofemoral joint, which play an important role in load transmission, shock absorption, and joint stabilization. The shape and root attachments of the meniscus allow it to have certain degree of mobility, and it can transform compressive loads and shear forces into hoop forces to protect the articular cartilage and subchondral bone from excessive load damage^[1^]. Normal meniscus function is integral to the prevention of joint degeneration, such as chondral lesions and osteoarthritis^[2^]. Conversely, meniscal injuries or degeneration may lead to a loss of hoop tension, potentially manifesting as meniscal extrusion (ME). This compromised functionality disrupts the efficient distribution of loads and the integrity of dynamic meniscal motion, thereby predisposing to chondral damage and hastening the progression of joint degeneration^[3^].

HIGHLIGHTS

• Persistent lateral meniscal extrusion was observed at 3 months postoperatively but showed significant gradual reduction at 6 months, with near-complete resolution at 1 year.

• Combined anatomical double-bundle ACL reconstruction and meniscal repair restored knee stability and alignment, facilitating a favorable healing environment.

• Early postoperative meniscal extrusion may not indicate surgical failure and can improve with time via biological remodeling and mechanical re-centering.

• Limb alignment and joint stability are critical for meniscal healing; physiological varus may offload the lateral compartment and support extrusion reduction.

Anterior cruciate ligament (ACL) injuries are predominantly caused by high-energy force, which can destabilize the knee joint, potentially leading to impingement, torsion, and compression of the menisci. It is documented in literature that up to 60–79.2% of ACL injuries exhibit concomitant radial tears, while 7–17% present with posterior lateral meniscus root lesions^[4,5^]. The disruption of the circumferential collagen fibers results in the loss of hoop stress, a condition that is significantly correlated with ME. Although favorable outcomes in terms of stability, kinematics, and function have been reported for meniscal repair with simultaneous ACL reconstruction (ACLR), a subset of research suggests that meniscal repair may not adequately correct the condition of extrusion. However, there is a paucity of research exploring the potential for a reduction of meniscus that remains protruded following repair surgery. In this case report, a patient’s meniscus, which remained protruded after lateral meniscal repair and ACLR, returned to its normal position without additional intervention at 1-year follow-up.

Case presentation

Initial presentation

A 23-year-old man complained of right knee pain after an injury during playing basketball, physical examination revealed signs of ACL and medial collateral ligament (MCL) rupture. MRI demonstrated multi-ligament injury, characterized by a complete rupture of ACL, with concomitant injury to MCL. Additionally, there are lacerations involving the body and posterior horn of the lateral meniscus, in conjunction with a lateral meniscal extrusion (LME) (Fig. 1 A, B, C, D).

Figure 1.

(A) ACL rupture; (B) MCL tear and lateral meniscus midbody tear; (C) Lateral meniscus posterior horn tear; (D) Lateral meniscal extrusion; (E) Lateral meniscus posterior horn repair; (F) Lateral meniscus midbody repair; (G) 3D femoral CT scan; (H) 3D tibial CT scan; (I) Preoperative MRI; (J) 3-month postoperative MRI; (K) 6-month postoperative MRI; (L) 12-month postoperative MRI; (M) Postoperative bilateral full-length standing anteroposterior X-ray of the limbs.

Arthroscopic operation

Arthroscopic evaluation confirmed the rupture of ACL, accompanied by radial tears in the body and posterior horn of the lateral meniscus. Subsequently, the radial tears of the lateral meniscus were adeptly addressed with the application of the inside-out technique (Fig. 1 E, F). Thereafter, the ACL was reconstructed using a double-bundle technique and MCL was repair using suture anchors (Fig. 1 G, H).

Rehabilitation protocol

The implemented rehabilitation protocol followed a phased approach: weight-bearing was restricted to non-weight-bearing for the first 4 weeks, progressed to partial weight-bearing from weeks 4 to 6, and advanced to full weight-bearing at week 7. Concurrently, the allowed range of motion was gradually increased from 0–45° during the initial 2 weeks to 0–90° in weeks 3–4, and further to 0–120° in weeks 5–6, with full range of motion exercises initiated at week 7. Subsequently, low-impact activities such as stationary cycling and swimming were introduced beginning at the 7-week postoperative milestone.

Follow-up

MRI evaluations were performed on postoperative 3, 6, and 12 months using a 1.5-T Siemens scanner and a coronal proton-density fat-saturated sequence (3-mm slice thickness). ME rate was quantified as a percentage, calculated by dividing the absolute extrusion distance by the total coronal width of the tibial plateau on the same slice. All measurements were performed by two experienced orthopedic surgeons; the operators were blinded to the time points. Serial MRI assessments demonstrated that at 3 months following surgery, the lateral meniscus persisted in its extruded position, with no notable amelioration relative to the preoperative status. However, at the 6-month mark, a gradual reduce of extrusion was observed, culminating in a near-complete reduction to its anatomical position by the 12-month evaluation (Fig. 1 I, J, K, L). Postoperative full-length of lower extremities anteroposterior X-ray examination revealed varus alignment in the lower extremities (Fig. 1M). The extent of ME and the corresponding knee functional scores are summarized in Table 1. Our work has been reported in line with the SCARE criteria^[6^].

Table 1.

The extent of meniscal extrusion and the corresponding knee functional scores.

Time point	Extrusion rate	VAS	ROM (°)	IKDC	Lysholm
Pre-op	0.088	5	0–60	36.8	57
3 Months post-op	0.083	1	130	43.7	78
6 Months post-op	0.043	1	132	57.5	78
1 Year post-op	0.018	0	135	66.7	95

VAS, Visual Analog Scale; ROM, range of motion; IKDC, International Knee Documentation Committee.

Discussion

Lateral meniscal injuries demonstrate a higher prevalence than medial injuries in the setting of ACL rupture, a distinction attributable to their distinct anatomical and biomechanical characteristics. Under axial load, medial ME typically increases, exacerbated by varus alignment which directs a greater proportion of force to the medial compartment. Conversely, LME tends to decrease under similar conditions. Biomechanical studies indicate that each degree of mechanical varus beyond 5° elevates the failure risk of medial meniscal posterior root repair, supporting the rationale for concomitant procedures like high tibial osteotomy (HTO) to offload the compartment and improve repair integrity^[7^]. Although HTO may not immediately correct extrusion, it can facilitate gradual meniscal re-centralization over subsequent months^[8^].

The clinical significance of early postoperative extrusion as a definitive predictor of surgical failure remains controversial. This case illustrates a delayed resolution of LME following a combined lateral meniscal repair and ACLR. While minimal improvement was observed at the 3-month assessment, a significant reduction in extrusion was evident by 1-year follow-up. This temporal pattern suggests that persistent extrusion in the early postoperative phase may not invariably signify repair failure but could reflect a transitional state of healing. A plausible explanation involves the gradual restoration of hoop tension, which is likely dependent on the maturation and reorganization of the repaired circumferential collagen fibers alongside the reestablishment of physiological joint kinematics and load distribution. This perspective is supported by emerging literature on meniscal healing potential, such as observations of fibrocartilaginous tissue formation following procedures like discoid meniscus saucerization combined with ACLR^[9,10^]. Proposed biological mechanisms facilitating this process may include healing responses mediated by mesenchymal stem cells from bone marrow stimulation sites or fibrin clot formation.

Lower limb alignment and the restoration of joint stability are critical factors creating a conducive environment for meniscal healing. In the present case, anatomical double-bundle ACLR likely enhanced anteroposterior and rotational stability. Furthermore, the patient’s physiological varus alignment may have contributed to a relative offloading of the lateral compartment post-surgery, thereby reducing mechanical stress on the healing meniscus. While previous research has established the biomechanical principles linking extrusion to lost hoop stress, there is a notable gap in longitudinal clinical studies documenting the natural history of extrusion following meniscal repair, particularly for the lateral meniscus and in relation to limb alignment. This case contributes to addressing this gap by providing serial imaging evidence of delayed LME reduction. It highlights that the healing process is dynamic and may extend beyond the early postoperative period. Future prospective studies with larger cohorts are warranted to validate these observations, identify factors (e.g., tear type, repair technique, alignment, rehabilitation) predictive of successful extrusion resolution, and clarify the long-term implications for chondroprotection. Such investigations would enhance clinical decision-making regarding the management of observed postoperative extrusion and the potential timing of secondary interventions.

Conclusion

This case suggests that residual early-stage extrusion might not definitively indicate surgical failure in all cases; regular monitoring is recommended. Restoration of joint stability and optimized load distribution could support the gradual regeneration of circumferential fibers and the reestablishment of hoop stresses, leading to a progressive reduction in extrusion over time.

Ethical approval

As this study involved only a retrospective analysis of anonymized patient images, formal ethics approval was waived by the Ethical Review Committee for Scientific Research of Fujian Medical University Union Hospital.

Consent

Written informed consent was obtained from the patient for publication and any accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal on request.

Sources of funding

This research was supported by Fujian Medical University Union Hospital Talent Launch Fund Project (No.2024XH017 and 2024XH032).

Author contributions

Z.C. wrote the manuscript, Z.C. and S.N. collected the data, and H.Z. designed this study and revised the manuscript. All authors read and approved the final manuscript.

Conflicts of interest disclosure

There are no conflicts of interest.

Research registration unique identifying number (UIN)

Not applicable.

Guarantor

Hua Zhang.

Provenance and peer review

Not commissioned; externally peer-reviewed.

Data availability statement

Data available on reasonable request.