Tibial avulsion fracture of the posterior root of the medial meniscus in a skeletally-immature child — A case report
Article Outline
Abstract
It has been theorized that a traumatic tibial avulsion fracture of the posterior root of the medial meniscus (MM) is the cause of the so-called meniscus ossicle (MO). We report the delayed appearance of a tibial avulsion fracture of the posterior root of the MM after a valgus, twisting injury in a 12-year-old boy with open physes. Magnetic resonance imaging (MRI) scans performed 3
days after the injury did not demonstrate a definitive tibial avulsion fracture of the posterior root of the MM; whereas, a repeat MRI for 3months post-injury did. Medial extrusion of the MM was also noted on the 3month MRI. Arthroscopic reattachment of the avulsed posterior root of the MM using a trans-physeal nonabsorbable suture tied over a proximal tibia staple was performed. Follow-up MRI at 6months postoperatively demonstrated healing of the tibial avulsion fracture of the posterior root of the MM in an anatomic position. The patient had a complete resolution of symptoms and there was no angular deformity or limb-length discrepancy at 2years postoperatively. To our knowledge, this is the first report describing a tibial avulsion fracture of the posterior root of the MM in a skeletally-immature patient successfully treated by a trans-physeal arthroscopic suture. This case also illustrates the development of the MO of the posterior root of the MM.
Keywords: Medial meniscus, Meniscal ossicle, Tibial avulsion, Posterior root, Open physis
1. Introduction
The meniscal ossicle (MO) was first described by Burrows in 1934 [1]. In 1995, Schnarkowski et al. [2] reported a 0.15% prevalence (six patients) of a MO among 1287 consecutive magnetic resonance imaging (MRI) examinations of the knee. Tibial avulsion fractures of the posterior root of the medial meniscus (MM) have been theorized as the origin of these ossicles as prior reports have described them as occurring in the vicinity of the posterior root of the MM following a traumatic injury [2], [3], [4], [5], [6], [7], [8], [9].
We report an avulsion fracture of the posterior root of the MM following a valgus, twisting injury in a 12-year-old boy that subsequently resulted in the formation of a MO based on serial MRI examinations. To our knowledge, this is the first report describing a tibial avulsion of the posterior horn of the MM in a patient with open physes and its successful treatment with a trans-physeal arthroscopic suture technique as has been done in adults. In addition, this case provides a plausible mechanism for the development of the MO. The patient and his parents were apprised that this case would be submitted for publication.
2. Case report
A 12-year-old male collided with a side safety barrier netting while snow skiing. His skis were caught in the netting, resulting in a valgus, twisting force on his left knee. He was admitted to the local hospital emergency room immediately for evaluation where it was determined that there was no significant musculoskeletal injury. He was subsequently referred to our clinic for further evaluation. On the physical examination, his left knee was moderately swollen; approximately 30 cc of bloody fluid was aspirated. He had mild tenderness along the medial joint line, but no pain or increased laxity to varus and valgus stress on the knee at 0° and 30° of flexion. There was a firm endpoint to the Lachman and anterior drawer testing, a negative pivot shift, and a normal stability to the posterior drawer testing at 90° of flexion. A KT-1000 evaluation was not performed. His neurovascular examination was intact to the left lower extremity and he had full, non-tender range of motion (ROM) in the left hip. Plain radiographs of the knee showed open femoral and tibial physes but no evidence of a fracture or dislocation. An MRI obtained 2days following the injury was significant for edema of the posterior horn of the MM, increased signal in the anterior cruciate ligament (ACL), and a mild joint effusion (Fig. 1). Our provisional diagnosis was a contusion of the MM and mild sprain of the ACL. Conservative treatment was initiated consisting of full weight bearing as tolerated, physical therapy emphasizing quadriceps and hamstring strengthening without limitation of ROM.
Fig. 1
Coronal fat suppressed proton density magnetic resonance (MR) image performed 3
days after the injury shows increased signal intensity in the MM initially diagnosed as meniscal contusion. The linear fluid interposition at the tibial insertion site of the posterior root of the MM is suspicious for a nondisplaced avulsion of the posterior meniscal root (arrow).
Three months post-injury, the patient did not have any complaints of pain, locking, catching, or instability. However, he described a sensation of “weakness” about the knee. On the physical examination, he had a symmetrical knee ROM and no evidence of increased knee laxity in the sagittal or coronal plane. He had a negative McMurray test, but complained of mild tenderness at the posteromedial joint line to direct palpation. Plain radiographs were, again, negative for any osseous abnormality (Fig. 2A). A repeat MRI revealed an avulsion of the posterior root of the MM with medial extrusion of the MM (Fig. 2B).
Fig. 2
(A) Anteroposterior and lateral radiographs performed 3
months post-injury show open femoral and tibial physes but no evidence of the tibial avulsion fracture of the posterior root of the MM. (B) But, coronal fat suppressed T2 weighted MR image performed 3months post-injury shows the extrusion of the MM with avulsion of the posterior meniscal root.
An arthroscopic evaluation was recommended due to the extruded meniscus with avulsion of the posterior meniscal root noted on MRI despite a paucity of symptoms. A classic MO was identified attached to the posterior root of the MM (Fig. 3A). The tibial defect was filled with a whitish fibrocartilaginous tissue, and the bony component of the avulsed fragment was identified after the removal of the fibrous scar tissue (Fig. 3B). There were no macroscopic abnormalities of the ACL, posterior horn of the MM or articular surfaces of the femur or tibia. The avulsed posterior root of the MM was fixed to its normal anatomic site using the modified method of arthroscopic reattachment for a complete radial tear of the tibial attachment site of the posterior horn of the MM (Fig. 3C) [10]. The 2 No. 2 FiberWire sutures (Arthrex, Naples, FL) were passed with a Caspari suture hook (Linvatec, Largo, Fl) in a simple vertical mattress configuration through the junction of the posterior horn of the MM and the avulsed bony fragment. The sutures were pulled through 6mm tunnels drilled through the proximal tibial physis using a wire loop, and secured to the proximal tibia with a bone staple (Arthrex, Naples, FL) (Fig. 3D).
Fig. 3
Intraoperative images of arthroscopic surgery. (A) The avulsion fracture of the posterior root of the MM with medial displacement. (B) The avulsed fracture fragment was covered with a fibrous scar tissue (MO) and the tibial defect was filled with a fibrous/fibrocartilaginous tissue. (C) Arthroscopic suture reattachment of the avulsion fracture of the posterior root of the MM to its normal anatomic site was performed. (D) The avulsed posterior root of the MM was fixed to its normal anatomic site with 2 FiberWire sutures tied over a bone staple distal to the proximal tibial physis.
Postoperatively, knee ROM exercises were begun immediately with a limitation of 90° of flexion. The patient was maintained in a derotation brace and kept non-weight bearing for 6weeks at which time partial weight bearing was initiated. Full weight bearing was allowed at 3months postoperatively with brace removal. Knee flexion beyond 90° and running were allowed at 6months postoperatively. Follow-up MRI at 6months showed that the reattached avulsed tibial fragment was maintained in its normal anatomic position (Fig. 4). At 1year postoperatively, the patient returned to normal sports activity, including soccer, without any symptoms. On both clinical examination and radiographic evaluations performed 2years after the injury, there was no evidence of angular deformity or limb-length discrepancy of the left lower extremity.
Fig. 4
Six-month postoperative coronal fat suppressed T2 weighted MR image demonstrates the reattached tibial avulsion fracture of the posterior root of the MM. It has remained in its normal anatomic position with normalization of the adjacent intrameniscal signal intensity.
3. Discussion
It is well known that tears of the posterior root of the MM are a relatively common lesion in Asian cultures due to their common practice of squatting during activities of daily living resulting in weighted hyperflexion of the knees [11], [12]. These tears are more commonly a result of chronic degenerative meniscal disease rather than a single traumatic event [13], and may be a precursor to the development of degenerative osteoarthritis of the knee due to meniscal extrusion [13]. A recent biomechanical study [14] evaluating the consequences of a posterior root tear of the MM demonstrated that it had similar effects as a total meniscectomy in terms of increased knee joint contact pressures and altered kinematics, and also reattachment of the posterior root was successful in restoring joint biomechanics to within normal conditions.
There have been only four case reports describing tibial avulsion fractures of the posterior root of the MM [3], [6], [8], [9]. In the first two cases, treatment consisted of immobilization [9] and meniscectomy [3]. However, in the latter two reports [6], [8], pullout suture reattachment of the avulsed fragment resulted in good results. To our knowledge, our case is the first report describing a tibial avulsion fracture of the posterior root of the MM in a skeletally-immature patient that was treated by arthroscopic reattachment. In our case, we used Kim's method, with some modification, for the reattachment of the fragment to its normal anatomical site by placement of the suture through the junction of the posterior root of the MM and the avulsed bony fragment [10]. We did not place the sutures directly through the avulsed fragment as it was felt to be prohibitively thin potentially resulting in a high risk of fracture during fixation.
It is generally accepted that the ACL of children has greater tensile strength than the femoral or the tibial physes and the adjacent bone [15]. Thus, injuries to the ACL in children most often result in an avulsion of the ligament from the anterior tibial spine rather than in rupture as is seen in adults [16]. We hypothesized that the relationship between the proximal tibial physis and the posterior root insertion of the MM is similar, such that a tibial avulsion of the posterior root occurred in this patient with open physes rather than a rupture of the posterior meniscal horn.
Our case is also unique in that it clearly shows direct evidence of the delayed development of a MO resulting from an avulsion fracture of the posterior root of the MM. MOs have historically been recognized as rare, unilateral ossified structures embedded in the posterior horn of the MM [2], [3], [8]. The exact etiology of these ossicles remains controversial [8], [17], though traumatic tibial avulsion fractures of the posterior root of the MM have been strongly implicated as the etiology in most reports [2], [3], [4], [5], [6], [7], [8], [9]. However, Yoo et al. [17] described a MO in the posterior horn of the MM that was not associated with an avulsion of the posterior root of the MM. The precise means by which these ossicles are formed may relate to both the nutrition of the articular cartilage as well as the meniscal tissue. Since the articular cartilage gains most of its nutrition from the synovial fluid [18], and the blood supply to the posterior horn of the MM is not disrupted despite the injury, growth of the ossicle (avulsed fracture fragment) may occur. This mechanism would explain the development of the MO after an otherwise unremarkable single traumatic event that was relatively asymptomatic.
A tibial avulsion fracture of the posterior root of the MM was clearly not shown on the original MRI examination obtained 3days post-injury in our patient. Only after a repeat MRI performed 3months following the injury did the avulsion (and formation of the MO) become evident. Despite this structural abnormality, the patient did not complain of significant mechanical symptoms suspicious for the diagnosis. In addition, the physical examination was relevant only for mild posterior medial joint line tenderness. In retrospect, the possibility of an incomplete or nondisplaced, complete tibial avulsion fracture of the posterior meniscal root should have been considered given the intra-substance signal change of the MM without any other significant lesions noted on MRI in the context of a traumatic, valgus twisting injury resulting in hemarthrosis in a child with open physes (Fig. 1). If a tibial avulsion fracture of the posterior root of the MM is suspected, absolute limitation of motion during the early healing of the injury is strongly recommended. It is possible that our either incomplete or nondisplaced avulsion fracture eventually displaced further through allowing protected ROM in the early post-injury period. Li et al. [19] reported that the knee is highly constrained at high degrees of flexion which could be due, in part, to the compression of the posterior soft tissues between the tibia and the femur. This constraint could result in elevated stresses in the posterior soft tissues at high flexion. This would explain the discomfort a patient with a meniscal injury feels when attempting to squat, whereby the injured meniscus is tightly compressed.
This case is also instructive in that it provides evidence that fixation of these avulsion fractures in skeletally-immature patients may be safely treated with arthroscopic fixation through trans-physeal tunnels using a nonabsorbable suture, as has been done in adults. This procedure resulted in adequate fixation, and subsequently, the resolution of symptoms without the creation of a limb-length discrepancy or angular deformity of the extremity. Though it may be argued that an absorbable suture may be sufficient for this type of repair without the potential “tethering” effect of a nonabsorbable suture on the proximal tibial physis, our patient did not demonstrate any evidence of growth plate arrest. This is likely secondary to the small diameter tunnel that was drilled through which the sutures were passed. We are unable to confirm whether or not the sutures ruptured at any point following their placement. Theoretically, it is possible that continued proximal tibial growth may have resulted in suture rupture after the fragment had healed. Nevertheless, this would be further evidence as to the benign nature of fixation used for this injury.
In conclusion, this is the first case, to our knowledge, describing a tibial avulsion fracture of the posterior root of the MM in a skeletally-immature patient successfully treated by arthroscopic trans-physeal suture fixation without the disruption of skeletal growth. This case also provides direct evidence as to the origin and development of the MO.
Conflict of interest
The authors, Young-Mo Kim and Matthew J. Matava, have not received any financial support and have no conflict of interest to report in the preparation of this manuscript.
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PII: S0968-0160(10)00008-6
doi:10.1016/j.knee.2010.01.007
© 2010 Elsevier B.V. All rights reserved.
