By: 9 September 2019
Focal resurfacing implants in the knee

Consultant orthopaedic surgeon Mike McNicholas presents an overview of the current evidence and treatment options available when considering resurfacing implants in the treatment of focal articular cartilage lesions in the knee

There is an increasingly demanding and active population presenting with persistently symptomatic knee pathology, which has not responded to conservative measures, who wish to continue their active lifestyle in comfort. While articular cartilage lesions of the knee are common, the majority are asymptomatic. If conservative measures fail in painful lesions or mechanical symptoms are evident then surgical intervention may be required. Techniques have been developed to treat the native knee with the aim of alleviating pain, preservation of physiological joint kinematics and preserving joint function for as long as possible.

Finding the best evidence-based surgical solutions for patients presenting with focal pathology within the knee can be difficult due to the multiple modalities of treatments available. Assisting patients in making shared decisions regarding their treatment is vital, an awareness of current national guidelines is important in the appropriate consenting of patients undergoing such surgery.

Adequate arthroscopic debridement is recommended as the initial surgical intervention, unless primary fixation of a loose osteochondral lesion is required.

If symptoms persist despite appropriate attendance and compliance in a post debridement rehabilitation programme, then the following surgical interventions may be employed: Bone marrow stimulation, osteochondral grafting, osteochondral scaffolds, chondrocyte cell therapy, stem cell therapies and osteochondral allografts. These have been discussed in detail in a recent paper by Biant and McNicholas.

Despite NICE guidelines suggesting cell-based therapies should be used for symptomatic articular cartilage lesions over 2cm2 published in October 2017, the ability of UK surgeons to comply with these guidelines is limited to a few centres. The commissioning process bringing other centres online is frustratingly slow. The success rates of most cell-based therapies are in the 80-90 per cent range for the correct indications.

Focal replacements can offer relief of symptoms as a primary treatment or secondary treatment of failed biological therapies. The advantages of focal resurfacing against biological solutions are that they allow immediate weight bearing and, potentially, offer a targeted long-term solution to full thickness osteochondral defects.

There are three main focal implants available in the UK market: the HemiCAP system was the first to market in 2003; over the past seven years, the Episealer and BioPoly systems have given further options in the orthopaedic surgeons armamentarium. This article presents an overview of the current evidence and treatment options available when considering resurfacing implants in the treatment of focal articular cartilage lesions in the knee.


Articular cartilage injuries and lesions of the knee are common, found in up to 61 per cent of knee arthroscopies. A report shows there are estimated to be about half a million per year identified on the 2.5 million arthroscopies in the USA. The majority are not symptomatic: a study of NBA players showed 7 per cent had incidental asymptomatic cartilage defects on MRI. Articular cartilage lesions over 0.9cm diameter will not heal spontaneously.

Articular cartilage defects can lead to a process of continued deterioration, but give us an opportunity, when symptomatic, to intervene and hopefully to prevent or at least delay secondary osteoarthritis. Symptoms from these focal traumatic cartilage lesions can be as severe and disabling as end-stage osteoarthritis and do warrant surgical treatment.

Surgical intervention is reserved for patients who have persisting symptoms after effective conservative interventions. As with all knee pathology, lifestyle modifications such as losing weight, analgesics, a targeted physiotherapy regime and patient education constitute first line treatment and this should be reinforced when one discusses treatment with patients as it forms the basis for post-operative knee rehabilitation.

Young patients presenting with a symptomatic knee who are in employment with high activity demands are a challenging cohort to treat and satisfy. Symptomatic improvement, preserving the native knee for as long as possible, while preventing further injury to the joint must be the goal of any treatment offered. Unicompartmental or total knee arthroplasty (UKA or TKA) in this group has always been approached cautiously, with concerns that treating a focal lesion with a total joint replacement should be considered overkill. Arthroplasty surgery, while a tremendous achievement, has its limitations, with a reduced survivorship in this group of patients when compared to the older population. The outcomes of knee arthroplasty with well preserved joint spaces are generally quite disappointing. Their use also commits both patient and surgeon to multiple revision knee surgeries. The population, as a whole, is living longer and so treatments for articular cartilage damage need to adapt to this with the aim of preserving as much of the joint as possible, delaying the point in time at which their first UKR or TKR is required. Medium-term results for the focal arthroplasties are encouraging and may help us achieve that aim.

Clinical presentation

Patients present with pain, mechanical symptoms and reduced function. These symptoms are often not specific to cartilaginous injury but indicate a disruption to the homeostatic environment within the knee. A thorough history and examination are essential. It is imperative to determine any instability or malalignment, as these will need to be addressed as part of the treatment. Weightbearing AP, lateral and skyline radiographs of the knee are the first line of investigation.

Assessing the mechanical alignment of the limb is important. If the knee is malaligned, this places abnormal forces on the articular cartilage. This puts the cartilage at risk of degeneration and decreases the likelihood of self-repair. Any implant will then be subject to these same abnormal forces. Focal arthroplasties could be combined with an unloading osteotomy if there is major malalignment documented. Long leg alignment radiographs provide accurate measurement of coronal alignment. Computerised tomography (CT) may be required to assess axial malalignment. Magnetic resonance imaging (MRI) has been found to be useful in defining lesions of the knee. It is important to appreciate the overall size, depth and location of any lesion within the knee prior to treatment and to ensure the lesion seen on scans correlate the patient’s symptoms with the findings and locations of lesions located on MRI, as some lesions may be asymptomatic.


Examination under anaesthesia should be done before performing arthroscopy, to obtain additional information about the stability of the knee, without causing any pain or discomfort to the patient. Arthroscopy allows both qualitative and quantitative evaluation of any cartilage defects and a more dynamic assessment. The grade, depth and size of the cartilage damage can be assessed. Concomitant intra-articular pathology can be identified and addressed.

The focal arthroplasties have a limited size range and it is important to ensure the size and depth of the lesion does not exceed the focal arthroplasty inventory, should the patient fail to settle with debridement.


Mats Britberg, in his talk at the recent One Step Cartilage Repair ICRS Focus Meeting held in Rome (5-7 June 2019) gave his indications for mini-metal implants as:

3rd line of treatment when other biological implants such as BMS or 3rd generation cell therapy has failed in chondral defects.

2nd line of treatment alternative after failed osteochondral repairs.

1st line of treatment of localised osteonecrosis with localised bone marrow oedema (Widespread bone marrow oedema could indicate fast progress of necrosis into full OA and local repair is then not indicated).

1st line of treatment in patients 45-60 years of age with lesions and degenerative changes before joint space narrowing.

I would add to his list that there are some patients who might ideally be treated with a biological technique, but whose work requirements do not allow the lengthy postoperative rehabilitation such techniques demand and for them focal implants can be used as a 1st line of treatment.

I would encourage anyone treating such patients to carefully study the outcomes of their interventions. There are industry-owned databases which companies supplying the implants may offer. The International Cartilage Regeneration and Joint Preservation Society (ICRS) have developed a user-friendly cartilage treatment registry that requires minimal input from the surgeon, and allows long term independent follow-up of this patient cohort which complies with all the relevant GDPR requirements and has the ability to generate reports suitable for appraisal requirements. It is available in eight languages; English, Japanese, Italian, Polish, Greek, Spanish, Portuguese and Dutch. Later this year, Swedish, Chinese and German language versions will go live. Visit

Implant surgery

The knee has three compartments: the medial and lateral tibio-femoral compartments and the patello-femoral compartment. Each of the manufacturers of focal arthroplasties have options to treat lesions in each compartment. Focal cartilage defects in the knee most commonly present on the femoral condyles, there are a variety of treatment modalities available, I will limit this article to discussing the management of femoral condyle lesions.

Any implants used in the knee must conform to several basic criteria. They must be tolerated within the human body, with no short-term and little long-term risk of adverse toxic effects. The aim must be to relieve pain and restore the activities of daily living. They must not interfere with the normal range of movement within the knee. They must be easy enough for the ‘average surgeon’ to insert and, in the few cases that fail, easy to remove or revise to another design or technique as osteoarthritis develops. Ideally, any implant would also be capable of use in combination with other joint preserving techniques.

Focal resurfacing implants

Due to the limitations of cartilage regenerative procedures, focal resurfacing implants have been developed to provide a more targeted solution than conventional arthroplasties and aim to ‘bridge the gap’ between these two treatment modalities.

Focal implants have been trialled in the past. The TRUFIT CB (a synthetic, resorbable implant plug from Smith & Nephew) was originally intended to backfill the donor sites after autologous mosaicplasty, but surgeons commenced using it directly into the condylar primary defects. This technique fell out of favour following a report that found good short-term results, but a reoperation rate of 70 per cent at two years follow-up where they found a cylindrical cavity of fibrous tissue rather than the subchondral bone they were hoping to produce. The system has ceased to be marketed. This experience highlights the need for responsible innovation and careful follow-up of techniques used.

There are a number of focal implants available on the market, in the UK, at present. HemiCAP, and Episealer are cobalt-chrome devices with hydroxyapatite on their undersurface. BiopolyTM RS Knee system is again uncemented, the articulating surface is a microcomposite of ultra-high-molecular-weight polyethylene (UHMWPE) and hyaluronic acid, creating a hydrophilic articulating surface with a material that has a closer modulus of elasticity to its surrounding biological tissue than the traditional metal bearing surface.

Clinical results of focal resurfacing implants

The literature regarding focal resurfacing implants is limited by small case numbers and short to medium-term follow-up.


The HemiCAP contoured articular prosthetic resurfacing prosthesis (Arthrosurface Inc., Franklin, MA, USA) was introduced in 2003 to offer a focal treatment option among the currently used modalities and close the treatment gap for the middle-aged patients. This consists of a cannulated cancellous screw with a tapered distal tip made of titanium alloy. This connects to an articular dome via a Morse taper. The dome is available in two diameters, 15 and 20mm. The articulating surface is a cobalt-chromium-molybdenum alloy with plasma spray coverage on the undersurface for bony on-growth.

In 2011, Becher et al produced their five-year follow up results of using the HemiCAP for medial femoral condyle defects. The patients’ mean age was 54 and there were 21 patients. They found an overall improvement in the KOOS score, as compared with their pre-operative scores, in all domains (pain 51.1 to 77.6; symptoms 57.9 to 79.5; ADLs 58.8 to 82.4; sports 26.3 to 57.8 and QOL 34.4 to 55.0.) The patient’s SF-36 physical health score improved from an average of 15.2 to 46.9, although the mental health component remained unchanged. 16 out of 21 patients were satisfied with the procedure.

In 2012, Bollars et al found similar results, reporting that 83 per cent of patients had normal, or nearly normal, post operative scores at an average follow-up time of 34 months. They also found that patients who did not meet their inclusion criteria fared poorly, with 7 out of 8 patients who had the procedure, but did not meet these, required conversion to a total knee replacement. The exclusion criteria were: age >65 years, BMI > 35, coronal plane malalignment > 7o varus or valgus, inflammatory arthropathy, chronic instability, greater than grade II changes in the opposing tibial surface, significant symptomatic damage to articular surface in other compartments and a lesion diameter of > 20mm.

The 2013 Australian joint registry reported their five-year revision rate for partial knee resurfacings (all HemiCAPs) was 26.4 per cent, representing 0.4 per cent of the Australian knee arthroplasty market. 86.4 per cent of implants were performed for osteoarthritis, 74.4 per cent of patients were under 55 years of age and there was a male majority at 54 per cent. Of the 176 implants, 125 (71 per cent) were femoral, 10 patella, 6 tibial, 7 trochlea and 2 unknown. The most common reasons for revision were disease progression (58.7 per cent) loosening (15.2 per cent) and pain (6.5 per cent). Most primary partial resurfacings are revised to either total knee replacement (54.3 per cent) or unicompartmental knee replacement (26.1 per cent). The remainder include revision to the patella or trochlear (8.7 per cent), addition of another resurfacing component (8.7 per cent) or removal of the prosthesis (2.2 per cent). This valuable lesson learned in the use of the first focal arthroplasty allowed a better understanding of the limitations of such a technique and has focused the subsequent use of focal arthroplasties to have the same indications as biological articular cartilage resurfacing technologies, limiting treatment to traumatic cartilage lesions rather than osteoarthritic ones.

A recent case report by Becher & Cantiller reviews two patients treated with HemiCAP implants  with a follow-up of 12 years and literature review comprising 6 studies with 169 focal articular prosthetic resurfacing procedures in 169 patients (84 male, 85 female) with a mean age at implantation ranging from 44.7 to 53.7 years and a follow-up range of 20 months to seven years. It suggested that focal articular prosthetic resurfacing is an effective and safe treatment option in selected cases. Re-operation rates ranged from 0 to 23 per cent depending on the length of follow-up. Meaningful clinical improvements and radiological safety were confirmed.

Episealer Mini Metal Implant (Episurf)

The Episealer implant is custom made cobalt chrome implant designed out of special MRI sequences via digital marker imaging which gives a comprehensive view of the total joint osteochondral situation and can be useful in preoperative planning if any metalwork from previous procedures will require removal.

The episealer has been studied in an adult sheep model where hydroxyapatite coated cap edges showed a better quality of tissue connection to the implant than those without. In another animal study, the importance of 0.5mm recession of the metal implant below the articular surface was required to avoid damaging the opposing tibial surface. Firm and consistent bonding to bone under weight-bearing conditions was shown up to one-year post operatively in a sheep model.

The first published clinical Episealer paper by Stålman et al. was a two-year prospective study of 10 patients aged 36-56 years with focal femoral condyle ICRS 3-4 chondral defects They had failed earlier conservative or surgical interventions and had VAS pain>40. Patient reported outcome measures (PROMS) used were VAS, EQ5D, KOOS and radiostereometric analysis (RSA). The customised implant and guide instruments were manufactured by computer-aided design/ computer-aided manufacturing (CAD/CAM) techniques using MRI data. Statistical and clinically significant improvements were seen in VAS (p≤0.001), Tegner (p = 0.034) and the KOOS subscores ADL (p=0.0048), sport and recreation (p=0.034) and quality of life (p = 0.037). VAS and KOOS scores improved gradually at 3, 6 and 12 months. The improvements in EQ5D, KOOS pain and KOOS symptoms did not reach statistical significance. No complications occurred in the postoperative period. No radiographic signs of damage to the opposing tibial cartilage was noted. The surgical usability of implants and instruments were good, and RSA did not show any implant migration.

BioPoly® RS

The greatest clinical experience I have with focal replacements is with the BioPoly implant. Along with Dinesh Nathwani, Alistair Hart, Jonathon Miles and Vladimir Bobic we prospectively studied a cohort of patients treated with BioPoly implants. BioPoly is a metal-backed implant with a patented surface combining hydrophobic ultra – high molecular weight polyethylene (GUR 1020) with hydrophilic hyaluronic acid (BioPoly RS). This non-biological implant offers a single stage management option for appropriately sized and located lesions. It has better wear characteristics than UHMWPE and a wettable surface, which interfaces well with articular cartilage. Defects up to 15 x 24mm or 3.1cm2 can be treated using the implant. Satisfactory success rates are reported in a short-term (2 year) study (NCT 01473199).

The BioPoly RS Knee Registry Study by Nathwani et al (2017) was a single-arm, prospective study in which 33 patients with focal cartilage lesions affecting the femoral condyle were managed with the BioPoly RS Partial Resurfacing Knee Implant. Knee injury and Osteoarthritis Outcome Score (KOOS) scores, a visual analog scale (VAS) for pain, the Short Form-36 (SF-36) physical component score, and the Tegner activity score were used to assess outcomes preoperatively and at 6 months, 1 year, and 2 years postoperatively. The KOOS outcomes at 2 years were compared with historical outcomes following microfracture treatment.

We found significant and clinically meaningful improvements in the KOOS scores, VAS pain score, and SF-36 physical component score (p < 0.025) when the values at all three postoperative time points were compared with the preoperative scores, and we also found significant improvements when the Tegner activity score at 2 years was compared with the preoperative score (p < 0.025). More than half of the cohort of patients had experienced a previous failure of cartilage-repair procedures. No significant differences were detected between younger patients (≤40 years) and older patients (>40 years). When compared with historical microfracture data, the BioPoly RS Implant demonstrated significantly superior KOOS scores for quality of life and sports.

We concluded that the BioPoly RS Partial Resurfacing Knee Implant is safe, that it resulted in significantly improved knee function by six months, and that this improvement was sustained for 2 years regardless of patient age. The BioPoly RS Knee Implant allows return to a higher level of sporting activity than microfracture. Additional long-term follow-up is needed to determine the long-term effects of the device.

More of our study patients have now walked through their two-year review and the results continue to show statistically and clinically significant improvement from baseline at all time points. To date, two of the 33 cases have required revision with biological techniques, with encouraging early results.

The first cases treated are now up to seven years follow-up and continue to have satisfactory outcomes. There have been more than 800 BioPoly implantations to date. Longer-term outcomes are awaited.


Focal articular cartilage lesions can be as disabling as an osteoarthritic knee. The majority of articular cartilage lesions, which remain symptomatic after appropriate conservative management can be helped by simple arthroscopic debridement. For those lesions that remain symptomatic after rehabilitation following that debridement there are a large number of surgical interventions available. Concurrent instability or malalignment of the knee should be addressed surgically. There are well established cell-based treatments with long-term follow-up that have been shown to be cost-effective, but frustratingly difficult to obtain for UK patients at present, despite NICE approval. For appropriate indications as discussed above, focal arthroplasty is available with encouraging early results. A significant advantage over biological treatments is that rehabilitation after focal arthroplasty implantation is less arduous and of shorter duration than with biological treatments. Both surgeons and patients should recognise the importance of adhering to proper indications for use of these implants and place appropriate expectations on these focal treatments. It is recommended that any such interventions should be followed up using an established Registry to allow comparative analysis and medium to long-term outcome assessments. As with all our interventions we need independent data to justify what we do, and to continually improve patient outcomes using evidence-based treatments. Longer-term outcomes are awaited with interest.


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Mike McNicholas

Consultant Orthopaedic Surgeon, University Hospital Aintree,

Liverpool, UK.