1 Introduction

A medial meniscus posterior root (MMPR) tear is a devastating knee injury that results in a functional loss of the medial meniscus comparable to a total meniscectomy.^1,2 If the quality of the meniscal tissue is sufficient, surgical repair of the MMPR tear is the preferred treatment to prevent the development of early osteoarthritis in the medial compartment.^2–5 Surgical repair of the MMPR tear is performed using either direct suture anchor repair or transtibial pullout repair.^6–9 In transtibial pullout repair, the sutures are passed through the MMPR and fixed to the tibia after being pulled through an anatomically positioned tibial tunnel.^10–12 Diverse tibial fixation techniques are described including the use of a bone bridge, a suture button or extraarticular anchor fixation.^9,11,13,14 The use of a bone bridge can be time consuming, potentially leading to unnecessary prolonged surgical time. In contrast, it does not cause potential soft tissue problems as reported in the suture button fixation technique (Fig. 1). However, in both techniques, it can be challenging to maintain the proper tension when tying the knot or securing the sutures at the tunnel entrance.^13,14 To overcome these technical limitations, the senior author (JR) developed a fixation technique where the MMPR loaded sutures are secured within the bone tunnel by inserting an interference screw while ensuring the appropriate tension arthroscopically. The objective of this retrospective study was to evaluate the transtibial pull-out repair technique utilizing tibial polyetheretherketone (PEEK) interference screw fixation for MMPR tear by reporting on short-term outcomes and complications.

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Fig. 1 Postoperative anteroposterior radiograph of a left knee after transtibial medial meniscus posterior root pull-out repair with tibial button fixation. Note that the button was not placed flush to the tibia. The patient suffered from pain at the tibial fixation side. After button removal the symptoms disappeared.

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2 Methods

The study was performed in line with the principles of the Declaration of Helsinki. The study protocol of this retrospective study was approved by the competent institutional research ethics board (OKM_MMPR_21) and all patients gave their informed consent to participate in the study and for publication. All consecutive transtibial MMPR pull-out repairs, performed between January 2019 and August 2021 were retrospectively included in the study (n = 70). Medical charts and surgical reports were analyzed regarding age at time of surgery, gender, body mass index, prior surgical procedures on the index knee, time from symptoms to surgery, documented Numeric Rating Scale (NRS) for pain, Lysholm Score and International Knee Documentation Committee Subjective Knee Form (IKDC), operation time, concomitant procedure, intraoperative chondral status according to International Cartilage Regeneration & Joint Preservation Society (ICRS), type of suture material and number of sutures, tibial fixation method and intra- (vessel/nerve injury, inability to perform repair) as well as postoperative complications (infection, thromboembolism, early failure of fixation) and rehabilitation protocol details. Further if available, preoperative radiographs were analyzed regarding joint degeneration according to Kellgren and Lawrence (KL) classification and coronal mechanical alignment of the leg was assessed in long leg standing radiographs.^15,16

The patients were contacted and followed for soft tissue problems on the tibial fixation side, revision surgery and meniscal re-tear. Further, clinical evaluation was performed using the NRS,¹⁷ Lysholm Score¹⁸ and subjective IKDC¹⁹ (patients with re-operation or re-tear were excluded from the clinical evaluation). Re-tear and repair failure were defined by the presence of symptoms (such as pain, locking, swelling …) and additional indications of re-tear/repair failure on magnetic resonance tomographs (MRI) or as intraoperative confirmation of re-tear or repair failure. In the case requiring revision surgery, the mode of failure was defined as meniscus (m) if the sutures were cut out from the meniscus, suture tear (st), if the sutures were torn between the MMPR and the tibial fixation and suture in situ but loose (sl), if the sutures lost their tension (Fig. 2).

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Fig. 2 (a–d): Arthroscopic view via the anterolateral portal into the medial compartment in four right knees during revision surgery after transtibial meniscus posterior root repair (a–d). The reasons of failure were (m) in a, the sutures were cut-out on the meniscal side; (st) in b and c, the sutures were torn between the meniscus and the tibial fixation side; (sl) in d, suture still in situ but loose.

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The impact of demographic parameters (age, gender, BMI and time from symptoms to surgery), surgical specifics (number of sutures) and rehabilitation details (duration of wearing an extension brace) on the occurrence of re-tear/failure and particularly mode of failure, as well as clinical Scores, was asessed.

2.1

2.1 Surgical technique

The leg was positioned in a leg holder without inflation of a tourniquet. Standard diagnostic arthroscopy was performed utilizing anterolateral and anteromedial portals. After verifying the indication for root repair two to three locking loops (No. 2 FiberWire®, Arthrex, Naples, Florida, USA) were placed into the medial meniscus posterior horn/root with a special suture device (FIRSTPASS MINI™, Smith & Nephew, Andover, Massachusetts, USA). Subsequently, a 4,5 mm tunnel was created at the anatomic insertion of the MMPR using a standard anterior cruciate ligament (ACL) aiming device. The FiberWire® sutures were passed through the tibial tunnel. While arthroscopically controlling the tension on the MMPR, a 7 × 25 mm polyetheretherketone (PEEK) screw (POWERFIX®, Karl Storz, Tuttlingen, Germany) was introduced into the tibial tunnel, positioned flush with the cortical bone of the tibia (Fig. 3). The sutures were tied together, creating a knot over the distal end of the PEEK screw to prevent possible early slippage. In cases (n = 3) were the surgeon identified soft bone during screw insertion, an additional hybrid fixation technique was used. The hybrid fixation consisted of an additional suture knot fixation onto a tibial post screw (small-fragment screw, 30 mm of length).

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Fig. 3 (a–c): Intraoperative arthroscopic images and postoperative MRI image of the same patient. (a) Arthroscopic transcondylar view (through the anterolateral portal and underneath the posterior cruciate ligament) of the medial meniscus posterior root (MMPR) insertion during MMPR repair. Two suture loops are placed into the MMPR and shuttled through the tibial anatomical placed tunnel. (b) The sutures are tensioned under arthroscopic control and fixed with a polyetheretherketone (PEEK) interference screw. (c) A sagittal T2 weighted magnetic resonance tomography image eight days postoperative shows the PEEK interference screw in the tibial tunnel.

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3 Results

62 of the 70 cases (88.6 %) were available for follow up after 2.3 ± 0.7 years (1.1–3.6). The demographic data is shown in Table 1. Intraoperative findings and surgical details are shown in Table 2. An extension brace was applied for two weeks in 11 patients (17.7 %), for four weeks in 14 patients (22.6 %) while 37 patients (59.7 %) used immidiate hinged knee brace allowing free range of motion.

Preoperative anteroposterior and lateral sided radiographs were available in 34 patients (54.8 %). In the medial tibiofemoral compartment the KL grade was 0 in 6, 1 in 12 and 2 in 16 patients. In the lateral tibiofemoral compartment the KL grade was 0 in 30 and 1 in 4 patients. In the patellofemoral compartment the KL grade was 0 in 15, 1 in 6 and 2 in 13 patients. Preoperative long leg standing radiographs were available in 16 patients (25.8 %): Mean 3.6° Varus ± 2.33° (3.4 Valgus – 6.3 Varus).

One patient (1.6 %) experienced soft tissue problems on the tibial fixation side (pain on the scar during direct contact), whereas 61 patients (98.4 %) had no problems. No revision surgery was performed related to problems with the tibial fixation material. During revision surgeries unrelated to the tibial fixation material, the PEEK interference screw was removed on the patient's request in five cases, without causing any problems (Table 3). In total, 13 patients (21.0 %) underwent revision surgery on the index knee after a mean of 0.7 ± 0.3 years (0.2–1.4), (Table 3). In ten cases the revision surgery was caused by a meniscal re-tear, while three were associated with other issues. Of these three revision surgeries, one was due to further cartilage deterioration caused by varus malalignment (>5°), another was attributed to loose meniscus suture material (resulting in squeak noises during knee motion) and the third due to extension loss caused by anterior cruciate ligament degeneration and subsequent notch-stenosis. During these three procedures the meniscal root demonstrated a complete and stable healing (Fig. 4). In summary, 11 patients (17.7 %) experienced a meniscus re-tear, among whom ten underwent revision surgery and one was managed conservatively. The mode of failure was (m) in seven patients (70.0 %), (st) in one patient (10.0 %), (sl) in one patient (10.0 %) and unknown (reoperation performed in another hospital) in one patient (10 %).

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Fig. 4 (a–d): Arthroscopic view via the anterolateral portal on medial meniscus posterior root (MMPR). (a) Torn root bevor suture placement during assessment with a probe, (b) after repair, (c) six months after repair during revision surgery because of loose suture material, (d) six months after repair during revision surgery, the assessment with the probe shows a complete and stable healing (suture material has been removed).

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The pre- and postoperative clinical Scores were availible from 38 patients displayed in Table 4, demonstrating an overall improvement. Age, gender, BMI, duration from symptoms to surgery and number of sutures showed no significant influence on re-tear incidence or postoperative clinical scores (all p = n. s.).

The analysis of extension brace duration revealed a non-significant trend towards higher re-tear rates with prolonged use of the extension brace (p = n. s.), (Table 5). However, patients with extension brace for two weeks demonstrated significantly better postoperative scores compared to patients without extension brace (Lysholm and IKDC) and showed a non-significant trend towards superior scores compared to patients with four weeks of extension brace (Table 6).

Further analysis of the influence on the mode of failure was not performed due to the limited number of patients in each group.

4 Discussion

The main finding of the current study was, that tibial PEEK interference screw fixation is a fast and promising technique for MMPR transtibial pullout repair. The most frequent reason of failure of the presented surgical technique, utilizing non-absorbable self locking sutures and tibial PEEK interference screw fixation, was suture cut-out from the meniscus.

The present study showed an overall significant improvement of all the assessed clinical scores even in this short-term follow-up of a heterogenic cohort. No intra- or direct postoperative complications were observed. However, a re-tear occurred in 17,7 % of the patients during the follow-up period. There are several studies reporting on healing rates after MMPR repair ranging from 90 up to 100 %.^10,12,20 These studies had strict inclusion criteria, including minimal excepted coronal malalignment, specific cut-offs for BMI and age, limited to acute tears and excluding higher grade chondral lesions.^10,12,20 In contrast, this study involved a heterogenous cohort where patients with chronic tears, high BMI, presence of grade 3 chondral lesions, and varus alignment were not excluded. Specifically, the negative influence of Varus alignment on the outcome after MMPR repair was previously well described.^3,10,21–24 However, Moon et al.²⁵ reported on comparable short term results after MMPR repair in patients with mild (<5° varus) compared to moderate (5–10° varus) varus alignment. In the present study only for 16 patients (25.8 %) a long leg standing radiograph was available, resulting in insufficient data to further explore the impact of coronal malalignment.

The primary reason of failure was suture cut-out from the meniscus. This may indicate that the meniscal tissue status, which deteriorated in time after injury, was a limiting factor in these cases. Notably, the mean time for symptoms to surgery in the present study was 15.7 ± 24.3 weeks. Additionally, a possible over-tensioning during screw insertion and the technique of suture placement throughthe MMPR may contribute to suture cut-out. However, several biomechanical studies reported minimal displacement and high ultimate load to failure in a technique similar to that utilized in the present study.^26–29 In the future, diverse suture materials may improve the resistance against suture cut-out. Nakama et al.³⁰ reported on a cadaveric study in 24 specimens, where they compared three different suture materials according to displacement and load to failure. The least displacement was found in the suture tape group. However, there was no difference in load to failure between the groups. Another option may be a placement of three instead of two sutures (as performed in seven cases in the present study). This was proposed by Camarda et al. after a biomechanical study in porcine tibiae, comparing two with three simple stitches.³¹ However, there was no difference in ultimate load to failure and only simple stitches, not loops were used.

There was no revision surgery or clear complication caused by the tibial fixation technique in the present study. However, one re-tear case showed sutures that were in place but loose and in two cases the suture was torn. It cannot be ruled out that this was due to tibial slippage or due to damage of the sutures during screw insertion. However, damage of the sutures is more likely to happen while passing the sutures through the meniscus with the sharp instrument. In three cases an additional tibial post screw fixation was used due to the impression of soft bone by the surgeon.

Therefore, bone density may be a limiting factor for the described fixation technique (e.g., in patients with osteoporosis). The tibial fixation in MMPR repairs was analyzed by Wu et al.¹⁴ They compared 4 different tibial fixation methods (transosseus suture, washer, extra-articular suture anchor and post screw with washer) in 24 porcine tibiae. Extra-articular suture anchor repair showed the least displacement after cyclic loading but lowest load to failure. A tibial interference screw fixation for MMPR transtibial pullout repair was already introduced by Okazaki et al.³² However, the technical aspects differed from those in the present study: simple stitches through the MMPR; controlled tension of 30 N during fixation, bioabsorbable interference screw (5- or 6-mm diameter) and additional tibial post screw for hybrid-fixation.

The rehabilitation protocol in the present study consisted of partial weight bearing for six weeks. This is consistent with most of the rehabilitation protocols which allowed increasing or full weight bearing after six to eight weeks.¹⁰ The present study revealed no differences in re-tear rates irrespective of the duration an extension brace was used (0, 2 or 4 weeks). The Lysholm and IKDC Scores were superior in patients with two weeks of extension brace compared to patients with none, but the results might be influenced by the varying follow-up times (extension brace was applied only until January 2020). Conversly, the immidiate postoperative motion could have led to a slightly elongated healing of the root, causing meniscal extrusion and consequently lower scores. Due to the limited number of patients in each group and the lack of follow-up MRI evaluation (evaluation of meniscal extrusion was not possible) the answer to this question remains unclear. No distinct restrictions were placed on the range of motion after the extension brace was removed or immediately postoperative if no extension brace was applied, this is in contrast to all other post-surgical protocols.^{10,11,33–35} Consequently, the results of the present study question the use of an extension brace for more than two weeks and the need for the restriction of range of motion until a distinct time point. In the opinion of the authors the possible and safe range of motion in rehabilitation is self-limiting due to pain and swelling. Interestingly, patients wearing a hinged knee brace allowing free range of motion immediately post-surgery showed a tendency toward a lower re-tear rate. However, these results are limited by the small number of patients and may be biased due to different follow-up times. Further prospective, high leveled comparative studies are needed to answer the question of optimal rehabilitation after MMPR repair.

The present study has several limitations that must be acknowledged. First, the retrospective setting with the well-known inherent limitations of such a design. Due to the lack of data the influence of coronal alignment was unknown. No preoperative MRIs were available to measure meniscal characteristics as meniscal extrusion and meniscal degeneration. Second, no follow-up MRI evaluation or second look arthroscopy was performed to assess meniscal healing and postoperative meniscal extrusion. Third, clinical scores were only fully obtained in 38 patients. However, the aim of the present study was not to report on influence factors for meniscal re-tear and clinical outcome but to report on the results and complications of the described technique.

This is the first study reporting on short-term results and reasons of failure after tibial PEEK interference screw fixation in MMPR pullout repair. The described technique, utilizing non-absorbable, self-locking sutures and tibial interference screw fixation, was fast (average operation time 33 min) and showed promising results. The short-term follow-up revealed good clinical results with an overall improvement in scores, even in a heterogenous cohort with negative prognostic factors for the outcome after MMPR repair.

5 Conclusions

Tibial interference screw fixation is a fast and promising technique for MMPR transtibial pullout repair. In the presented technique, utilizing non-absorbable locking sutures alongside tibial interfenrence screw fixation the primary mode of failure was suture cut-out from the meniscus.

Funding information

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Ethical approval

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. The study protocol was approved by the competent institutional research ethics board.

Consent to participate

All patients gave their informed consent to participate in the study.

Consent to publish

The authors affirm that the patients provided informed consent for publication of their data and images.

Informed patient consent

Written informed consent has been obtained from the involved patients or if appropriate from the parent, guardian, power of attorney; and, they have given approval for this information to be published in this case series.

Complete written informed consent was obtained from the patient for the publication of this study and accompanying images.

CRediT authorship contribution statement

Michael Schlumberger: Conceptualization, Methodology, Data curation, Writing – original draft. Stefan Michalski: Investigation, Writing – review & editing. Wouter Beel: Data curation, Visualization. Philipp Mayer: Writing – review & editing. Philipp Schuster: Writing – review & editing, Data curation. Micha Immendörfer: Investigation. Raul Mayr: Writing – review & editing, Conceptualization. Jörg Richter: Supervision, Methodology.