1.1 Inclusion and exclusion criteria. The inclusion criteria were (1) age < 55 years, (2) pain on the medial side only, (3) varus angle < 5°, (4) injury of the posterior part of the medial meniscus, (5) arthroscopic exploration showing narrowing of the medial space in the semi-straight position of knee, the posterior aspect of the medial meniscus was not clearly visible, (6) need to perform release of the PMC of the knee, and (7) postoperative follow-up time of more than 1 year. The exclusion criteria were (1) degenerative injury of the meniscus, (2) Outerbridge grade of cartilage degeneration ≥ 3 grade, (3) combined ligament or lateral meniscus injury, (5) previous knee operation, and (6) inflammatory arthritis. This study was approved by the Hospital Ethics Committee, and informed consents were obtained from all patients.
Based on aforementioned criteria, 60 patients with posterior injury of the medial meniscus between January 2016 and January 2018 were included in this study. The patients underwent pie-crusting release of the PMC under arthroscopy to improve the field of view of the posteromedial space of the knee. Patients comprised 32 males and 28 females with an average age of 37.0 ± 5.3 years (16-55 years). The mean body mass index (BMI) was 25.0 ± 2.1 kg/m2 (21.5–31.4 kg/m2). Thirty-eight patients had injury to the left knee and 22 to the right knee. The mean time from injury to surgery was 35.5 ± 2.1 days (7-120 days). There were 26 cases with only posterior meniscus injury, and 34 patients with posterior plus body injury. Meniscoplasty were performed in 40 patients, and suturing was performed in 20 patients. According to the O'Connor classification of meniscus injury, there was a simple horizontal tear in 20 knees, a sagittal tear in eight cases, a longitudinal tear in 20 knees, and a complex tear in 12 knees. According to the Outerbridge classification of cartilage injury, 22 cases were grade 0, 21 were gradeⅠ, and 17 were grade Ⅱ [8]. The clinical symptoms were locking, clicking, joint swelling and claudication. All operations were performed by a single surgeon.
1.2 Surgical technique. A load cell with capacity of 50 kg and accuracy of 0.01 kg (model L6D, ZEMIC Ltd., Hanzhong, Shaanxi, China) was mounted on the lateral block post of the operating table to monitor the amount of valgus stress that was applied to the patient’s thigh during the operation. Under general or spinal anesthesia, the patient was positioned supine on the operating table with a tourniquet at the proximal thigh. Standard anterolateral (AL) and anteromedial (AM) approaches were used to examine meniscus, cartilage and ligaments. For patients with medial meniscus injury, the exposure of the posteromedial compartment was observed through the AL approach with the knee joint in a semi-straight position at 20°. Valgus and external rotation stress of 11-kg was applied to the leg by the surgeon using his body [9]. When the medial compartment was narrow, the posterior part of the medial meniscus was not clearly visible, and it was difficult to smoothly insert the blue forceps into the posterior space. Then outside-in controlled multi-point pie-crusting release of the MCL and POL were performed. The detailed procedures were as follows: (1) The probe hook was inserted into the medial joint space through AM approach, and the width of the posteromedial gap was evaluated with reference to whether the probe tip (4 mm height; Smith & Nephew plc., London, UK) could pass through the narrow medial space vertically (Fig. 1a). (2) At the near level of the medial joint line, percutaneous multi-point pie-crusting piercing of the posterior part of the MCL and the POL with an 18-G venous needle was carefully carried out after identifying the course of the saphenous nerve and vein using transillumination with the arthroscope, and it is appropriate to sense a "slight tear" when piercing the aforementioned tissues (Fig. 1b). (3) Under 11-kg valgus stress, multi-point release stopped when the posteromedial space was suddenly opened. At this time, the posterior part of the meniscus could be clearly viewed, and the probe tip could easily pass through the narrow space in the vertical orientation (Fig. 1c). (4) When the posteromedial space was opened satisfactorily, meniscoplasty or suture could be performed.
1.3 Postoperative management. After recovery from anesthesia, isometric contraction of the quadriceps femoris and straight leg elevation training were performed. Patients who underwent meniscoplasty were recommended to wear an short knee brace for 4 weeks to prevent further injury of the MCL, and were allowed to do full-weight bearing and full range of motion exercise two days after operation. Patients who underwent meniscal suture were encouraged to practice ambulation without weight-bearing on the affected limbs under the protection of a adjustable brace and crutch. Four weeks after operation, patients carried out partial weight-bearing activities, then gradually the brace was removed and full weight-bearing activities were allowed six weeks after operation, with active knee flexion more than 120°. Stationary cycling and moderate intensity running were allowed 3 months after surgery, and full return to sport was permitted 6 months later.
1.4 Arthroscopic measurements of the joint space width (JSW). The intraoperative width of the medial space before and after release of the PMC was measured as follows. A probe hook was inserted through the AM approach, and its vertical tip was placed against the lowest point of the femoral condyle cartilage at the narrowest space. Before release, the vertical tip (4mm) often formed an acute angle with the articular surface; The width of the narrowest space (h1) can be calculated by the sine formula (h1= 4 mm × sin A), where A is the angle between the vertical tip and the articular surface (Fig. 2a). After release, the medial space was enlarged, and the vertical tip could pass vertically through the narrow space, and the opening width of the medial gap (h2) can be calculated by the formula (h2 = 4/H1 × H2 + 4 mm), where H1 is the magnified measured height of the vertical tip of the hook under microscopy (greater than 4 mm), and H2 is the magnified measured height between the tip of the hook and the lowest point of the femoral condyle under microscopy (Fig. 2b). h1-h2 is the extra opening width. The height of H1 and H2 was measured with a digital calliper with accuracy to 0.1 mm.
1.5 Radiographic measurements of the JSW. The identical load cell was mounted on the stress radiography device to monitor the amount of valgus stress that was applied to the patient’s thigh while performing X-ray films. With the patient in supine position and the knee in a semi-straight position at 20°, an anteroposterior (AP) knee radiograph with 11-kg valgus stress of the lower leg was taken with the X-ray beam centred in the joint line and a tube distance of 1 metre from the cassette. JSW was measured using a picture archiving and communication system (PACS) as follows with modification [9]. On an AP X-ray film, a line was drawn to connect the subchondral bone of the medial and lateral tibial condyle. From this line, a perpendicular line was drawn to the most distal point of the medial femoral condyle. The distance of this vertical line was recorded as JSW and measured using PACS software (PACS, PI View STAR, version 5025; Infinitt, Seoul, Korea) (Fig. 3).
1.6 MRI analysis of healing of the meniscus and MCL. All MRI examinations of the knee was performed using a 3.0-T system before and after operation. A special superficial sagittal section was taken to evaluate the localization of the PMC injury. In coronal sections, the localization of the injury was recorded with reference to the medial meniscus [9]. Based on MRI signal grade, the meniscus was considered completely healed if there was no fluid signal within it, incompletely healed if an intrameniscal signal approached only one articular surface, and unhealed if the signal hyperintensity extended from one articular surface to the other [10].
1.7 Clinical evaluation. To evaluate the effect of arthroscopic pie-crusting release during operation, the width of the medial space before and after release was measured; the improvement of the visual field and occurrence of iatrogenic cartilage injury were also observed. During the follow-up, the medial stability of the knee joint was evaluated by radiographic measurements of the JSW preoperatively, and at 1 week and 3 months postoperatively. Healing of the MCL and sutured meniscus was evaluated by MRI. VAS [11], Lysholm score [12], Tegner score [13], and IKDC score [14] were used to evaluate knee joint functions.
1.8 Statistical analysis. The measurement data are expressed as mean ± standard deviation. Student’s paired t-test was used to test the significance of differences, and P < 0.05 denoted a significant difference. All statistical analyses were performed with SPSS 15.0 (SPSS Inc., Chicago, IL, USA).