Focal resurfacing of the knee

Abstract

In the past decade, three distinct systems for addressing focal cartilage lesions in the knee through resurfacing have emerged, namely the Hemicap/Unicap, BioPoly and Episealer systems. These technologies provide a new approach for treating patients who might not be suitable candidates for other biological treatment options. Specifically, they target individuals considered too old to benefit from biologic repair methods, but who are also too young for a unicompartmental knee arthroplasty. The three systems have demonstrated promising clinical outcomes, with the literature showing that they can effectively relieve pain and restore knee function, with success rates comparable to other established treatments for cartilage injuries. An additional advantage of these surface replacement systems is their relatively short rehabilitation period, which allows for a quicker return to daily activities. In conclusion, they are an ideal option for ‘gap-age’ patients, representing a valuable alternative for managing knee cartilage defects in this specific patient cohort.

Introduction

Over the past 30 years, there has been significant research and development focused on the treatment of focal cartilage lesions in the knee. Biological treatments, such as cell-transplants, also known as autologous chondrocyte implantation (ACI), or a variety of artificial scaffolds have been added to the orthopaedic arsenal, along with bone and cartilage graft procedures such as osteochondral autografts (OATS) and allografts.

Nevertheless, the treatment of focal lesions of the knee remains controversial. The age of the patient is important, with Minas et al. reporting that ACI survival is negatively affected by patient age when a relatively low threshold of 30 years was applied. At the other end of the age spectrum is joint arthroplasty, which is usually reserved for patients aged 60-plus. Hence, an age gap has been identified, between approximately 35 years and 65 years of age, where focal resurfacing could potentially be an appropriate alternative option for some patients.

Focal lesions are rather common, and they do not heal spontaneously. They should rather be considered as an early stage of a slow biologic process that starts with early trauma and ends with total knee replacement (TKR) after decades, and after considerable pain and functional impairment for the patient.

Overloading of the cartilage edges around a focal lesion has been implicated in the centrifugal breakdown of cartilage, but this ‘pot hole effect’ is not often discussed. In the context of surface replacements, this may be particularly relevant. There are enormous mechanical loads across the knee joint, often exceeding four to five times body weight. The load on the surrounding cartilage has been shown to be related to the size of a lesion, and the larger lesion, the larger edge loading. For an untreated defect (i.e. with no implant) with a diameter of 12 mm, the edge load increases by approximately 50%. Importantly, unlike some other modes of treatment, small hard focal resurfacing implants are able to support the surrounding cartilage and may represent a unique way of slowing/stopping at least one pathway towards further development of osteoarthritis.

Using small metallic implants to resurface focal articular cartilage lesions offers a novel treatment approach. Currently, there are three systems available on the market: the HemiCap/Unicap (Arthrosurface Inc. Franklin, Mass, USA), the Episealer (Episurf Medical, Stockholm, Sweden) and the BioPoly (Schwartz Biomedical, Fort Wayne, Ind, USA).

Indications

Proper indications for surgery are always imperative. Factors of importance in the context of focal resurfacing in the knee include the status of the surrounding cartilage, which should be of sufficient height to allow counter-sinking of the resurfacing implant. The opposing cartilage should be normal, or only minimally degenerative. ICRS Stage III and IV cartilage damage on the opposing joint surface constitutes a contraindication for resurfacing surgery with unipolar systems.

In orthopaedics, pain is usually the most significant symptom bringing a patient to a doctor. With focal cartilage lesions, the pain does not emanate from the cartilage itself, but from the subchondral bone, and this can be seen on MRI as bone marrow lesions (BMLs). Hence, a BML adjacent to the femoral cartilage lesion predisposes for successful surgery, while a BML in the opposing tibia may indicate a potential contra-indication to surgery.

The menisci play a crucial role in distributing forces across the cartilage surfaces, which is why it is essential for the surgeon to ensure that the menisci are functional. This includes the surgeon ensuring that the anterior and posterior insertional ligaments of the meniscus are intact, and that there is no meniscal extrusion.

All three systems emphasize that the implants must be recessed below the surface of the surrounding articular cartilage. This in turn necessitates a standard standing radiograph in order to ascertain that there is next to normal cartilage around the lesion. The condition of the underlying bone should be considered. If using the Episealer, the implant can be made thicker to fill underlying cysts, or the cyst can be bone grafted prior to placement of the implant.

For the Episealer system ( Figure 1 ), the potential suitability of a case is determined with the help of a preoperative MRI, which is used to produce a Damage Marking Report (DMR), ( Figure 2 ). The shape of the knee, the size and shape of the lesion, as well as the status of the surrounding and opposing articular cartilage, menisci, ligaments and, importantly, the subchondral bone, are all assessed. The DMR is subsequently further used for design of the implant and the instrumentation.

For the Hemicap and BioPoly, final indications for their use are often determined at the time of the operative procedure (arthroscopy or arthrotomy). With a suitable cartilage lesion, the size and shape of the implant is immediately chosen from pre-manufactured units and inserted.

Contraindications are stated in the respective ‘Instruction For Use’ of each system. Inflammatory joint disease is an absolute contraindication. Smoking, as well as relevant metabolic disorders and excessive body mass index (BMI) should be addressed. Usually, conservative management, such as physiotherapy, is suggested as an alternative for these patients.

Specific anatomy/biomechanics

Focal resurfacing implants function quite differently from the biological articular cartilage grafts in a number of ways. First, the missing surface is replaced by these implants, although with varying degrees of precision. It follows that joint kinematics are immediately restored to normal, with some variation between the three systems. For the Episealer, the kinematics of the knee has recently been shown to be next to normal.

Second, the cartilage edges around an untreated defect become overloaded, leading to degeneration. One advantage of focal implants is that, once implanted, they provide immediate support to the surrounding cartilage. This is a unique effect of hard material focal implants, distinguishing them from other treatment methods.

Third, while cartilage is not innervated, bone is. By excising the BML often found in the subchondral bone beneath a cartilage defect, the pain in the knee typically resolves very quickly.

The Episealer system

The Episealer implant is a one-piece cobalt-chrome device with a double coating, hydroxyapatite (HA) on-top-of-titanium (Ti) ( Figure 1 ). Various sizes are available in either circular diameter, from 12 mm to 25 mm, as well as figure-of-eight shape, up to 2 × 25 mm. The thickness of the implant is down to 3 mm, and is adjusted depending on the shape of the distal femur and the depth of the defect.

Using MRI, the knee is reconstructed into a three-dimensional (3D) image, where the femoral cartilage and any lesion is defined. Opposing cartilage, bony defects and BMLs are assessed, as well as meniscal and ligamentous integrity. The size, thickness and position of the implant are designed based on the anatomy and size of the defect.

Following a small arthrotomy, a patient-specific guide is placed on the cartilage lesion and pinned in place. The guide facilitates the correct position of the implant, and after drilling the proper counter-sinking of the implant approximately 0.5 mm below surrounding cartilage, the preparation is verified using a dummy implant ( Figure 3 ). If needed, the drill depth can be adjusted by drilling in increments of 0.2 mm using a dedicated drill socket. Immediate fixation is achieved by press-fitting the peg into a slightly undersized drill hole.