Unicompartmental Knee Replacement

Understanding Knee Compartments and Localised Arthritis

The knee is commonly described as having three compartments:

• Patellofemoral compartment (behind the kneecap)
• Medial compartment (on the inner side of the knee)
• Lateral compartment (on the outer side of the knee)

Arthritis may involve one, two, or all three knee compartments. In some people, wear is predominantly limited to one side of the joint. This is often due to:

• Long-term load patterns or leg alignment
• Previous meniscal injury or meniscal removal
• Prior ligament injury
• Previous fracture or surgery
• Gradual age-related cartilage thinning

When only one compartment is significantly affected, a total knee replacement may not always be required. Preserving the healthier compartments and ligaments may help maintain a more natural feeling in the knee, depending on your situation.

Symptoms Suggesting Single-Compartment Knee Wear

You may have unicompartmental arthritis if you notice:

• Pain that tends to localise to the inner or outer side of the knee
• Stiffness after rest that eases as you start moving
• Swelling that worsens with longer walks or time on your feet
• Discomfort when walking on slopes, uneven ground, or stairs
• A feeling that your knee is “loading” more heavily on one side
• Reduced ability to walk, kneel, or squat comfortably

Symptoms often build gradually and may fluctuate. Some people describe a pattern of better and worse days depending on activity and load.

How Unicompartmental Arthritis Is Diagnosed

Diagnosis aims to confirm that arthritis is localised and that the rest of the knee, including ligaments, remains suitable for a partial replacement. This usually involves:

• Clinical examination: Assessment of leg alignment, ligament stability (especially the ACL), range of motion, and areas of tenderness.
• X-rays: Standing X-rays help show how the joint space looks under load and whether narrowing is limited to one compartment or more widespread.
• MRI (in selected cases): MRI may be used to assess cartilage surfaces, meniscal status, and the condition of surrounding structures, particularly if there is uncertainty about the pattern of wear.

The decision to consider unicompartmental knee replacement is based on this overall picture rather than imaging alone.

When to See a Knee Specialist

You may benefit from seeing a knee specialist in Sydney if you notice:

• Ongoing pain focused on one side of the knee despite physiotherapy or medication
• Difficulty walking the distances you need for work, family, or daily life
• Stiffness or swelling that limits stairs, inclines, or uneven ground
• Reduced confidence in your knee when pivoting or changing direction
• Uncertainty about whether non-surgical options are still reasonable

A thorough review helps clarify whether ongoing non-surgical management, unicompartmental knee replacement, or total knee replacement is more appropriate for your situation.

Non-Surgical Options Before Considering Surgery

Surgery is rarely the first step. Many people are managed for a period with non-surgical strategies that may help reduce symptoms and delay or avoid the need for an operation. These may include:

• Physiotherapy to strengthen the quadriceps, hamstrings, gluteal, and calf muscles
• Gait retraining and movement coaching to reduce excessive load on one side
• Activity modification, such as choosing lower-impact exercise or adjusting workloads
• Weight management strategies when appropriate
• Simple pain relief or anti-inflammatory medication if suitable for you
• Joint injections, where clinically indicated
• Bracing or orthotics in selected cases to offload the affected compartment

If these measures no longer provide adequate relief and your symptoms affect your quality of life, it may be appropriate to discuss surgical options with an orthopaedic knee surgeon.

When Unicompartmental Knee Replacement May Be Considered

Unicompartmental knee replacement is usually considered when:

• Arthritis is mainly limited to one compartment
• The ligaments, including the ACL, are functioning and stable
• The remaining compartments have relatively preserved cartilage
• There is no significant fixed deformity that cannot be corrected
• Pain and stiffness significantly affect daily activities despite non-surgical care

Partial knee replacement aims to:

• Replace only the damaged surfaces
• Preserve more of your bones and ligaments
• Maintain more natural knee kinematics where possible

Not everyone with knee arthritis is suitable. Some people are better served by a total knee replacement, particularly when arthritis is more widespread or the knee is unstable. Dr Jonathan Negus will discuss the reasons why a particular option is or is not recommended in your case.

Robotic-Assisted Partial Knee Replacement

Robotic-assisted systems are used in some unicompartmental knee replacements to support planning and component positioning. In Dr Negus’s practice, two systems may be used when appropriate:

Mako Total Knee SmartRobotics™

• Uses CT-based imaging taken before surgery
• Creates a 3D model of your knee to support personalised planning

VELYS™ Robotic-Assisted Solution

• Does not require a pre-operative CT scan
• Creates an intra-operative model of your knee during surgery
• Allows real-time adjustment based on soft-tissue balance and alignment

These technologies are tools that may help support precision. They do not replace the surgeon’s judgement or guarantee a specific result. In Dr Negus’s practice, there is no additional cost for robotic knee replacement surgery at participating hospitals, although your overall costs still depend on the hospital, your insurer, and your individual level of cover.

What Happens Before Surgery

Before any knee replacement, it is important to confirm that surgery is appropriate and that you are medically optimised. This may involve:

• Pre-operative consultation to review imaging, discuss options, and answer questions
• Blood tests and other investigations as required
• Review by your GP or physician if you have conditions such as heart disease, diabetes, or lung disease
• Medication review, including guidance on stopping or adjusting blood thinners, anti-inflammatories, or certain supplements
• Planning for support at home, including help with shopping, cleaning, or transport, in the early phase
• Education about what to expect in the hospital and during rehabilitation

If you smoke, reducing or stopping smoking before surgery may help lower the risk of complications and support wound healing.

What Happens on the Day of Surgery

On the day of surgery, you are usually admitted to the hospital either that morning or the night before, depending on hospital arrangements and your health needs. You can expect:

• Pre-operative checks with the nursing staff and your anaesthetist
• Marking of the surgical side and final confirmation of the procedure
• Cleaning of the skin around the knee with an antiseptic solution in the operating theatre

Unicompartmental knee replacement is usually performed under general or spinal anaesthesia, sometimes with a nerve block to support early pain control.

How Unicompartmental Knee Replacement Is Performed

During surgery:

• A measured incision is made at the front of the knee to access the affected compartment
• The worn cartilage and a thin layer of underlying bone from the damaged compartment are carefully removed
• The bone surfaces are shaped to accommodate the prosthesis
• Metal components are placed on the femoral (thigh bone) and tibial (shin bone) sides, usually with a polyethylene (plastic) insert between them to allow smooth, low-friction movement
• The remaining compartments, ligaments, and soft tissues are preserved as much as possible
• Alignment, stability, and motion are checked before closing the incision and applying the dressing

The duration of surgery varies based on your anatomy, the pattern of joint damage, and whether robotic technology is used.

Right After Your Operation

After surgery, you are taken to the recovery area where staff closely monitor your comfort, breathing, circulation, and pain control. Once stable, you return to the ward. In the early phase, you can generally expect:

• Pain relief through a combination of tablets, injections, or nerve blocks, as appropriate
• Measures to reduce blood clot risk, such as stockings, early movement, and medication, where suitable
• Guidance from nursing staff on safe transfers in and out of bed
• Early exercises to promote circulation, muscle activation, and joint mobility

Many people begin standing and walking with assistance within the first day or two, under the supervision of a physiotherapist, provided it is safe to do so.

Rehabilitation and Recovery After Partial Knee Replacement

Recovery timelines vary from person to person. Your rehabilitation program is usually progressive and may include:

Early phase (first few days to 2 weeks)

• Weight-bearing as advised by your surgeon
• Gentle range-of-motion exercises
• Quadriceps and hip strengthening
• Swelling management with elevation and ice when appropriate

Intermediate phase (2 to 6 weeks)

• Progression of walking distance and speed
• Continued strengthening, balance, and gait retraining
• Gradual return to light daily tasks and office-based work if suitable

Later phase (6 weeks onwards)

• Ongoing strength and endurance training
• Low-impact conditioning such as cycling, walking, or hydrotherapy
• Gradual return to recreational activities as advised

Many people aim to achieve comfortable walking and functional bending by around 6 to 12 weeks, although this varies. Results depend on factors such as pre-operative function, general health, and adherence to rehabilitation.

Long-Term Outcomes and Activity Expectations

A unicompartmental knee replacement is designed to last many years, but it is still an artificial joint. Long-term outcomes can be influenced by:

• Activity level and type of sport or work
• Body weight and overall health
• Underlying bone quality and ligament function
• Wear of the plastic insert over time

Low-impact activities such as swimming, walking, cycling, and golf are generally more favourable for joint longevity than high-impact or contact sports. If arthritis progresses in other compartments, or if wear or loosening occurs, revision to a total knee replacement may be considered in the future.

Results may vary between individuals. Regular follow-up with your surgeon helps monitor the joint and address any concerns early.

Risks and Possible Complications

Every operation carries potential risks, which your surgeon and anaesthetist will review with you thoroughly. Potential risks after unicompartmental knee replacement can include:

• Medical complications such as heart or lung problems, infection, or blood clots
• Wound issues, including delayed healing or irritation around the scar
• Stiffness or reduced range of motion
• Persistent pain or discomfort despite surgery
• Infection in the joint, which may require further surgery and prolonged antibiotics
• Blood clots can form in the legs (deep venous thrombosis) or travel to the lungs (pulmonary embolism)
• Fracture of the bone around the implant
• Wear, loosening, or failure of components over time, which may require revision surgery

Although serious complications are uncommon, they are possible, and understanding them is an essential part of informed consent.

Next Steps

If you have ongoing pain mainly on one side of the knee and are unsure whether a partial or total knee replacement, or continued non-surgical care, is most appropriate, a detailed assessment can help clarify your options.

You can arrange a consultation with Dr Jonathan Negus, an orthopaedic knee surgeon in Sydney, to discuss your symptoms, imaging, and the full range of suitable treatment pathways.

This information is intended as a general guide only and may not apply to every situation. Assessment and treatment recommendations vary depending on individual circumstances. Please speak with a qualified health professional for advice specific to your condition.