The first-generation anatomical medial meniscus prosthesis led to unsatisfactory results: a first-in-human study

This study was set up as a first-in-human, prospective, multi-centre, open-label, single-arm clinical investigation and approved for 18 patients by the local medical ethical review board (METC Arnhem-Nijmegen number: 2017–3988) and registered in the Dutch Trial Register (NTR: NL75393.000.21). A written informed consent was obtained for each patient in the study.

Patient population

Three Dutch hospitals were involved in the study (Radboud University Medical Centre, Nijmegen (RUMC), Sint Maartenskliniek Nijmegen (SMK) and Maastricht University Medical Centre (MUMC)). In Fig. 1 the screening flow diagram is shown. In total, 5 patients underwent a medial meniscus replacement with an artificial meniscus prosthesis between February and September 2019.

Fig. 1figure 1

Patient screening flowchart. Of the planned 18 patients, only 11 were included at the moment of termination of the study

Surgeries were performed by 2 experienced knee surgeons (KD, PE). The most important inclusion criteria and exclusion criteria are listed in Table 1. The entire list with inclusion and extrusion criteria is published on www.toetsingonline.nl (NL64121.091.17).

Table 1 Inclusion and exclusion criteriaArtificial meniscus prosthesis

The synthetic polymer meniscal implant (ATRO Medical BV, Nijmegen, The Netherlands) (Fig. 2) was developed to treat patients with a painful medial knee compartment with a deficient meniscus. A statistical shape model was created, containing the meniscus geometries of 35 subjects (20 females, 15 males) that were obtained from MR images [17]. Based on this model an average geometry meniscus prosthesis could be composed and based on a finite element model [6]. Five different sizes for both the left and right knee (L1–5, R1–5) could be injection-moulded. Medical-grade PCU was used for the meniscus prosthesis, allowing a flexible two-component construct with a softer variant PCU in contact with the cartilage and a stiffer reinforcement ring in the core of the implant to resist the circumferential forces and mimic the circumferential strength of the native meniscus. However, this reinforcement ring made the prosthesis significantly stiffer in the horizontal plane on the tibial plateau than the native meniscus. This means that the prosthesis may move less easily with the femoral condyle than the native meniscus. The meniscus prosthesis is attached to the tibia plateau solely by an anterior and posterior horn fixation. The prosthesis is clicked onto 2 titanium screws, specifically designed for this procedure. The screws are placed in the tibial plateau at the native meniscus horn location. The prosthesis requires no peripheral fixation to the capsule. In case the prosthesis has to be removed, only two 3.2 mm drill holes remain, which quickly close again (Fig. 3).

Fig. 2figure 2

Artificial medial meniscus prosthesis

Fig. 3figure 3

Artificial meniscus prosthesis with screws for fixation in the tibia

Surgical technique

After anaesthesia, knee stability was evaluated by the surgeon to confirm a stable knee joint and to assess the preoperative range of motion of the knee joint. The procedure started with a standard arthroscopy and the indication for the meniscus prosthesis was confirmed. When the indication was confirmed, the remnants of the medial meniscus were removed. The medial parapatellar vertical incision was extended to approximately 5 cm in length. An aiming device and trial sizers were designed specifically for this procedure. With the trial sizers, the size of the meniscus prothesis was determined based on the following landmarks: the eyelets of the trial sizer should be located on the anatomic meniscus horn attachments and the contour of the trial sizer should follow the tibial plateau edge.

A 3.2 mm Steinmann pin was drilled upwards under arthroscopic control and emerged through the posterior eyelet of the trial sizer, exactly on the posterior anatomic location of the horn of the native meniscus. The screw hole was tapped and the specially designed posterior fixation screw was placed until the neck of the screw was fully visible in the back of the knee. For placement of the anterior screw, the trial sizer was left in place with the eyelet around the posterior screw. The exact location of the anterior drill hole was determined by rotating the trial sizer around the posterior screw. The specially designed anterior screw was placed. Subsequently, the meniscus prosthesis was placed above the screw heads and the posterior horn of the meniscus prosthesis was pushed onto the screw head and followed by the anterior horn of the prosthesis.

Rehabilitation

The rehabilitation protocol was developed specifically for this procedure and started with 4 weeks of partial weight bearing (50%) with mobilization on crutches and increasing the load based on pain and swelling. No brace or immobilization was applied. This partial weight bearing was required to allow the implant to become saturated with water (~ 1%) before full loading to ensure that the maximum flexibility of the prosthesis is achieved. Because the prosthesis is somewhat stiffer in the plane of the tibial plateau and therefore less mobile during flexion and extension, the material must additionally adapt by creep. This creep occurs when the patient puts weight on the implant [15]. When the knee joint achieved a full range of motion, all restrictions were abandoned, except the advice to avoid high-impact activities and sports. Given the early stage of development of this new meniscus prosthesis, it is not yet known how long the implant can withstand these high loads while practising sports.

Follow-up

Primary outcome for this preliminary analysis was Knee injury and Osteoarthritis Outcome Score (KOOS) Pain, assessed at baseline, 3, 6, 12 and 24 months. In addition, KOOS symptoms, Activities of Daily Living (ADL), Sports and Recreation, Quality of Life (QoL), the Lysholm score and the IKDC score were assessed. Weightbearing radiographs on the preoperative screening were taken and at 12 and 24 months to evaluate screw position and fixation. MRI scans (metal artifact reduction sequence) were repeated after 12 and 24 months to evaluate implant integrity and to evaluate the status of the cartilage via the modified Outerbridge grading scale. Other secondary outcomes also included adverse device effects (ADEs) during 24 months of follow-up. Due to implant removal in four out of five patients not all data could be collected.

Statistical analysis

The primary endpoint was the performance of the meniscus prosthesis, measured as a decrease in the KOOS Pain subscale. The minimal clinically important change (MIC) of the KOOS is 8–10 points [14]. When an MIC of 8 is applied calculating the sample size (mean difference) and a standard deviation of 10 points (based on pain improvement of an alternative meniscus prosthesis) [12], the size of the group should be 15 patients. To compensate for a dropout percentage of 20% (loss to follow-up, incompleteness of data, complications with surgery or rehabilitation), the sample size was set at 18.

All (available) patients were followed over time and the patient-related outcome measures (PROMs) and other outcomes are presented with descriptive statistics such as median and interquartile ranges.

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