Robotically assisted unicompartmental partial knee replacement offers younger, active patients with knee osteoarthritis a joint-preserving, more natural alternative to total knee replacement—with a faster return to activity and no radiation exposure. The CORI system enables reliable implant positioning with millimeter-level precision, making this procedure a highly effective option with considerable, as yet untapped, potential.
Studies show that up to 50 percent of all patients with knee osteoarthritis would be suitable candidates for a partial knee replacement—yet worldwide, only about ten percent actually receive one. The editorial team of the Leading Medicine Guide spoke about this with Priv.-Doz. Dr. med. Philipp A. Michel, M.Sc., specialist in orthopedics and trauma surgery with a focus on endoprosthetics and robot-assisted surgery.
Since July 2025, PD Dr. Michel has been practicing at the Center for Hip and Knee Endoprosthetics at ETHIANUM Heidelberg, which he leads together with Prof. Dr. med. Jörg Holstein. He previously served as Senior Physician and Head of the Endoprosthetics Section at University Hospital Münster (Department of Trauma, Hand, and Reconstructive Surgery), holds a postdoctoral lecturing qualification, and has published extensively in the scientific literature.
When the knee starts defining everyday life – the path to diagnosis
Knee osteoarthritis rarely begins with a dramatic moment. It is more of a gradual process: first, climbing stairs becomes painful; then jogging routes get shorter; eventually, even walking the dog becomes difficult. When conservative measures have been exhausted and quality of life declines, the question of joint replacement arises.
„The focus is usually on very basic limitations in everyday life: pain when walking, problems climbing stairs or getting in and out of the car, often accompanied by increasing restriction of movement because the knee can no longer be fully straightened or bent. In some cases, there are also problems falling asleep or staying asleep.
By this point, many patients have already been through a long history of treatment—with physical therapy, pain medication, injections (cortisone, hyaluronic acid, PRP), and other conservative measures that eventually are no longer enough“, PD Dr. Michel describes this typical course of suffering.
Because damaged cartilage in osteoarthritis still cannot regenerate, joint replacement remains the only curative option. The first step involves a fundamental decision: Is a complete resurfacing replacement necessary—in other words, a prosthesis that replaces the entire knee joint—or is a partial replacement sufficient?
„Such partial prostheses may, for example, involve only the inner part of the joint (the medial compartment). However, there are also forms of osteoarthritis that primarily occur on the outside (the lateral compartment) or behind the kneecap (the retropatellar compartment). When people talk about a ‚partial knee replacement,‘ they are usually referring to replacement of the medial compartment“, explains PD Dr. Michel.
When is a partial knee replacement an option?
Not every arthritic knee requires complete joint replacement. If the wear is truly limited to a single compartment and the remaining parts of the joint are still healthy, a unicompartmental partial knee replacement may be an option—a significantly more tissue-sparing procedure that largely preserves the knee’s natural feel of movement.
A stable anterior cruciate ligament is essential, because it controls the rolling-gliding motion that remains fully preserved with a partial replacement. The collateral ligament guidance must also be intact.
„When a patient first comes in, the main priority is really to take the time to get to know the person in front of you. At ETHIANUM, we devote a full hour to every patient during the initial consultation.
Which symptoms are limiting everyday life? Which hobbies matter most? Should the knee make long hikes possible again, relaxed cycling, maybe even golf or tennis? Or is the main goal, at an older age, to be able to walk short distances without pain? The demands vary enormously. Only once it is clear who is actually sitting in front of us and what this person needs do we move on to the physical examination and review of the findings they have brought with them“, says PD Dr. Michel.
The diagnostic work-up
After the detailed consultation, the actual diagnostic process begins: clinical examination, ultrasound, and review of existing X-rays. If necessary, high-resolution MRI scans are obtained, showing the condition of the cartilage in all compartments with precision. Studies show that MRI diagnostics achieve a sensitivity of more than 94 percent for severe lateral cartilage damage—significantly more than conventional weight-bearing X-rays alone.
„During the examination, many patients report pain that mainly affects the medial compartment. For the final decision, additional special X-rays are needed: standing images to measure the leg axis, stress views to assess ligament stability, and further images showing patellar tracking. For efficient evaluation, we rely on AI-based analysis from ImageBiopsy Lab. Based on these criteria, we assess step by step whether the patient is a suitable candidate for a partial knee replacement. The key approach is this: we generally consider the possibility of a partial replacement openly and positively from the outset. Everyone who walks through the door is initially regarded as a potential candidate—unless the diagnostic findings show clear reasons against it“, explains PD Dr. Michel.
AI-supported measurement of the leg axis on full-length standing X-rays using the IB Lab LAMA module from ImageBiopsy Lab. This allows bowlegged or knock-kneed alignment to be determined reliably and quickly.
Anatomical requirements in detail
For a medial partial knee replacement, several anatomical requirements must be met so that the procedure makes sense and the prosthesis remains stable over the long term: „It is crucial that the cartilage in the lateral compartment is still sufficiently preserved. The joint behind the kneecap should also be largely unremarkable—although minor cartilage damage, as is almost always seen on MRI in people over 65, does not rule out a partial knee replacement.
The osteoarthritis must be clearly limited to one compartment (Figure 2). Ideally, the anterior cruciate ligament should be stable. In recent years, however, some flexibility has emerged here: in older patients with secondary cruciate ligament insufficiency, a partial knee replacement may still be an option, because certain deficits can be compensated for through the robotically precise adjustment of the prosthesis inclination. These criteria—defined in part by the Oxford group and currently being further refined by the AE (German Society for Endoprosthetics)—help reliably identify suitable candidates“, PD Dr. Michel explains.
At a glance – Requirements for a partial knee replacement: isolated medial (or lateral) compartment osteoarthritis with bone-on-bone contact, preserved cartilage in the opposite compartment, functionally intact anterior cruciate ligament and medial collateral ligament, correctable leg-axis deformity (varus up to 10–15°), no inflammatory arthropathy.
Typical presentation of anteromedial osteoarthritis in a 75-year-old patient. On the X-ray, the loss of joint space in the medial compartment is visible. During surgery, the missing cartilage and already worn bone in the medial part of the joint can be seen. Age-appropriate cartilage coverage is present in the rest of the knee joint (lateral and retropatellar compartments).
Great potential, limited use – why partial knee replacement is still performed so rarely
In Germany, only around 14 percent of affected patients currently receive a partial knee replacement (unicompartmental knee arthroplasty, or UKA)—even though studies show that 41 to 49 percent of all candidates for total knee arthroplasty (TKA) would actually be suitable for a partial replacement. There is therefore a considerable gap in care.
„Why is partial knee replacement used so rarely? One major reason is the high level of technical precision this surgery requires. The bone cuts have to be made within the millimeter or even submillimeter range, and ligament tension must be assessed with exact precision. Small errors can cause a partial knee replacement to fail prematurely.
A surgeon who performs around one hundred knee replacements a year will statistically only do about fifteen partial knee replacements—too few to build the necessary routine. But because they are performed so rarely, they are also taught less often, and many young specialists do not feel confident performing the procedure after training. It is a vicious cycle“, PD Dr. Michel analyzes.
Registry data confirm this problem: surgeons with less than a five percent UKA share per year achieve the poorest five-year survival rates (90 percent), while high-volume surgeons performing more than 30 partial knee replacements annually reach up to 96 percent. There is also a reimbursement issue: in the German DRG system (Diagnosis Related Groups), which governs hospital reimbursement, payment for a partial knee replacement is often lower than for a total knee replacement—despite comparable or even higher technical demands.
„It is also important to bring office-based colleagues on board. Many still rely on older registry data in which partial knee replacements had higher revision rates than total replacements. That often leads to counseling that essentially goes like this: ‚With a partial knee replacement, you will have surgery twice; with a total knee replacement, only once.‘ When patients come to the consultation with that impression, it is difficult to shift their perspective.
This is where education is needed—and above all, good outcomes. Ultimately, satisfied patients are the strongest argument“, emphasizes PD Dr. Michel.
Preserving more instead of replacing less – the functional advantages
For younger, active patients, the advantages of a partial knee replacement are easiest to convey by showing that this is not about “less” replacement, but about preserving more of their own functioning knee joint. The evidence from studies has now become impressive:
Particularly revealing is the so-called Forgotten Joint Score: patients with UKA “forget” their artificial joint significantly more often—a sign that the knee feels much more natural than after a total knee replacement. Walking speed also returns to the level of healthy control subjects after a partial knee replacement (2.2 m/s), whereas it remains measurably reduced after a total knee replacement.
These observations are impressively confirmed by a current Level I study: in a study published in the Journal of Bone and Joint Surgery (2026), Mortensen and colleagues compared medial partial knee replacement directly with total knee replacement for the first time in a double-blind, multicenter randomized controlled trial involving 350 patients across ten centers.
In the Forgotten Joint Score, there was a clinically relevant difference of 14.1 points in favor of the partial knee replacement, exactly matching the threshold at which patients actually notice the difference in everyday life. Knee mobility was also significantly better, with seven degrees more flexion, as were knee-specific symptoms and pain scores in overall quality of life. It is also noteworthy that the reoperation rate after partial knee replacement was significantly lower at 2.3 percent than after total knee replacement at 6.9 percent—a finding that casts common registry analyzes in a new light.
„What makes the partial knee replacement special is that the ‚return to sport‘ is much better than with a total knee replacement—around 90 percent of patients return to their previous level of sports activity. That is exactly what makes it so attractive for many active patients“, PD Dr. Michel summarizes.
A prominent example is elite ski racer Lindsey Vonn, who received a robotically assisted lateral partial knee replacement in her right knee in April 2023 using the MAKO system. In November 2024, she announced her comeback—at the age of 40. In the 2025/26 season, she won two World Cup downhill races before a severe crash at the Olympic Games in Cortina ended her comeback.
Prof. Wassilew (AE) puts this into perspective: the case demonstrates impressively what is medically possible—even though it cannot serve as a benchmark for standard care, and the risk of injury in elite sports exists independently of the prosthesis.
How robotics is revolutionizing precision – the CORI system
Robotic assistance with the CORI Surgical System (Smith+Nephew) is fundamentally changing surgical decision-making—even before the first cut is made. Instead of relying exclusively on preoperative X-rays and the surgeon’s own experience, the surgeon receives an accurate three-dimensional representation of the individual knee during the procedure.
CT-free: less radiation, less effort
One major advantage of the CORI system compared with other robotic systems such as MAKO (Stryker): it works entirely without a preoperative CT scan. The 3D bone model is created directly during surgery through surface mapping with a handheld probe and reflective markers. The ATRACSYS infrared camera system captures the position more than 300 times per second.
„For the patient, this means: no additional CT appointment, no radiation exposure, and no waiting for a radiology appointment. The path from indication to surgery becomes faster and easier“, explains PD Dr. Michel.
Haptic boundary technology: precision to 0.5 millimeters
The core of the CORI system is the hand-guided robotic burr—not a robotic arm, but an instrument controlled directly by the surgeon. The system monitors its position in real time and automatically stops or retracts the burr if the planned boundaries are exceeded. The documented accuracy is 0.5 mm and 0.5° in all three planes.
„The plan established beforehand can be implemented precisely in the OR, without outliers and without unpleasant surprises later on the X-ray. The dynamic gap balancing is particularly valuable: the system captures soft-tissue tension throughout the entire range of motion in real time—before any bone resection. This makes individually tailored balancing possible in a way that is hardly achievable manually. That creates safety and expands the range of options“, describes PD Dr. Michel.
3D mapping of the joint surfaces followed by bone preparation using the high-speed burr.
The evidence base underscores the clinical relevance: in a prospective comparative study, 100 percent of robotic implants were within 2° of the planned position, compared with only 40 percent using conventional technique. The outlier rate for posterior tibial slope drops from 25 percent (conventional) to less than 4 percent (CORI/NAVIO).
Learning curve – the key advantage: conventionally, it takes 25 to 50 partial knee replacements before a surgeon achieves reliable precision. With the CORI system, this learning curve is reduced to just 5 to 6 procedures in terms of operating time—and implant accuracy is within the target range from the very first case. For the first time, this also enables lower-volume surgeons to achieve excellent results.
What patients can concretely expect – surgery, recovery, sports
CT-free 3D imaging and robot-guided preparation offer patients a number of concrete advantages: less blood loss, less pain in the first weeks, and faster recovery. Bone resection is more tissue-sparing because the burr works only within the stereotactically planned volume—robotic procedures require significantly fewer aggressive soft-tissue releases.
„Let us take the example of a 65-year-old recreational golfer—a profile I know well from my own experience. After a partial knee replacement, he remains in the hospital for about three to four days. Rehabilitation is not absolutely necessary because the implant can bear full weight immediately. When leaving the hospital, he is safely mobile with forearm crutches.
After two to three weeks, many patients already stop using the crutches. A follow-up examination takes place after six weeks, and if everything looks good, there is little standing in the way of returning to sports. Between the eighth and twelfth week, most patients can start golfing again—initially on the driving range before attempting a full round“, PD Dr. Michel explains.
Comparison of X-rays of a medial partial knee replacement (left) and a total knee replacement (right). The lower invasiveness of UKA becomes clear through preservation of healthy cartilage and bone, as well as the cruciate ligaments.
„Sports involving hard impacts or rapid changes of direction are generally not recommended, but they are certainly possible—and far better tolerated than with a total knee replacement. Golf, cycling, hiking, skiing, or tennis are all easily possible. Regular exercise, stable musculature around the knee, and good overall fitness support joint function and relieve stress on the prosthesis over the long term“, advises PD Dr. Michel.
Limits and risks – an honest assessment
„As convincing as the advantages of partial knee replacement are—it is not the right solution for every patient. Transparency about limits and risks is part of responsible counseling. There are clear contraindications: inflammatory arthropathy such as rheumatoid arthritis, severe flexion contracture, fixed varus deformity, or relevant ligament instability.
Old injuries to the medial collateral ligament may also be an exclusion criterion. And anyone who shows pronounced wear across all three compartments of the knee needs a total knee replacement—the best robot cannot change that“, PD Dr. Michel makes clear.
PD Dr. Michel offers a differentiated assessment of the long-term results: „The major prosthesis registries show higher revision rates for partial knee replacements than for total knee replacements. However, these data reflect the average across all surgeons—including many low-volume surgeons. High-volume surgeons performing more than 30 partial knee replacements annually achieve ten-year survival rates of 97.5 percent—comparable to the best TKA results worldwide.
In addition, the threshold for revision is lower with a partial knee replacement: conversion to a total knee replacement is a comparatively straightforward procedure, whereas revision of a failed total knee replacement is far more complex. The previously mentioned study by Mortensen et al. provides important context here: in the primary endpoint—the Oxford Knee Score—the difference between partial knee replacement and total knee replacement was statistically significant, but at 3.5 points it remained below the threshold of clinical relevance.
That means: in the general functional score, both procedures perform similarly well. The decisive advantage of the partial knee replacement lies in the secondary endpoints—joint awareness, mobility, symptoms—and in the lower complication rate. So the point is not that the partial knee replacement is superior in every respect, but that in the properly selected patient it delivers the more natural knee. For the majority of patients with knee osteoarthritis, total knee replacement remains the right and well-established treatment“.
The concept at ETHIANUM – state-of-the-art technology in a personal setting
At the Center for Hip and Knee Endoprosthetics at ETHIANUM Heidelberg, around 400 prostheses are implanted each year—hip and knee prostheses combined. With 30 to 40 percent, the share of partial knee replacements is well above the national average.
„What matters less than the sheer number of procedures is the concept behind them. The priority here is to devote sufficient time to each individual person—in conversation, preparation, surgery, and follow-up care. The entire process remains in one set of hands. We do not perform ten joint replacements a day, but only as many as can be carried out responsibly with care and personal attention“, PD Dr. Michel describes the center’s philosophy.
Knee and hip from a single source: robotics meets AMIS
The center follows a holistic approach to endoprosthetics of the major joints. In addition to robotically assisted knee surgery with the CORI system, hip replacements are performed using the AMIS technique—a minimally invasive anterior approach that follows a similar principle: modern technology, high precision, and therefore tangible benefits for patients.
„Recovery after hip surgery using AMIS is often even faster than after knee surgery—most patients can be discharged after just three to four days. The combination of minimally invasive hip replacement, high-quality implant systems, and robotic support for knee replacements creates an offering that is medically state of the art while taking place in a calm, personal environment.
Through cooperation with Salem Hospital in Heidelberg and St. Josefs Hospital in Viernheim, we can offer the AMIS technique to all patients, regardless of insurance status“, adds PD Dr. Michel.
The technology concept at ETHIANUM: robotically assisted knee arthroplasty (CORI Surgical System) for total and partial replacements, minimally invasive hip arthroplasty (AMIS technique), high-quality implant systems—in a private hospital setting with individualized, personal care.
Outlook – how more patients can benefit
For more people to benefit from a partial knee replacement, a shift in thinking is needed on several levels: in education and training, in determining indications, and in reimbursement structures.
„There are already committed pioneers who teach in courses and workshops that significantly more patients are eligible than many assume. They encourage surgeons to start with straightforward cases and allow their own experience to grow. Because once you see how satisfied this patient group is, you quickly gain confidence in the procedure. When people are remarkably mobile again after just a few weeks and bring that experience back to referring practices, it changes perceptions“, PD Dr. Michel is convinced.
The German Society for Endoprosthetics (AE) already recommends more than 30 UKAs per surgeon per year and a UKA share of more than 20 percent per hospital. It is currently developing a clinical practice recommendation for UKA indications. Together with the increasing spread of robotic assistance systems, which drastically shorten the learning curve, the partial knee replacement could achieve its deserved standing in knee endoprosthetics in the coming years.
Many thanks, PD Dr. Michel, for this well-founded and nuanced discussion of the opportunities and limitations of robotically assisted partial knee replacement!
Priv.-Doz. Dr. med. Philipp A. Michel, M.Sc.
- Specialist in Orthopedics and Trauma Surgery, Special Trauma Surgery, Center for Hip and Knee Endoprosthetics at ETHIANUM Heidelberg (Prof. Holstein / PD Dr. Michel)
- Focus areas: knee and hip endoprosthetics, robot-assisted surgery (CORI)
- Previously: Senior Physician and Head of the Endoprosthetics Section at University Hospital Münster, Department of Trauma, Hand, and Reconstructive Surgery (2013–2025)
- Postdoctoral lecturing qualification on tendon, ligament, and meniscal injuries of the lower extremity
- 30+ original articles, 10 review articles, 3 book chapters
- Member: DGOU, DGU, AGA, AO, DKG, AE
Prof. Dr. med. Jörg Holstein
- Specialist in knee and hip surgery as well as endoprosthetics
- Focus: minimally invasive techniques (AMIS, DAA)
- At ETHIANUM Heidelberg since 2018; previously almost 15 years at Saarland University Medical Center (including Deputy Medical Director)
- Specialist in Orthopedics and Trauma Surgery with numerous additional qualifications
- Active in professional societies and widely published in the scientific literature
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