Dr. Sebastian Rodriguez takes a deep dive into partial (unicompartmental) knee replacement—covering knee anatomy, who's a candidate, surgical techniques with robotics, and what to expect during recovery.
The knee joint has three major compartments: the medial side (inside part), the lateral side (outside part), and the patellofemoral joint (where the patella/kneecap articulates on the end of the femur).
The bones involved include the femur (thigh bone), the tibia (shin bone), and the patella (kneecap). All three bones can become arthritic.
Ligaments & Stabilizers
There are two major ligaments that provide side-to-side stability: the medial collateral ligament (MCL) and the lateral collateral ligament (LCL). These are often injured in sports.
Additionally, the ACL and PCL (anterior and posterior cruciate ligaments) provide front-to-back stability.
The meniscus also provides stability—it's a bumper of cartilage that increases the surface area of contact between the femur and tibia. Both bones are coated with hyaline cartilage, a smooth Teflon-like coating that allows smooth articulation.
How Isolated Compartment Arthritis Develops
Valgus vs. Varus Alignment
Everyone has their own natural alignment. Some people are very straight, while others have:
Valgus alignment (knock-kneed) — Associated with more erosion on the lateral (outside) part of the knee
Varus alignment (bow-legged) — Puts pressure on the medial (inside) part of the knee, the most common area for knee arthritis
This alignment is largely genetically transmitted—if someone is significantly bow-legged, their parents likely have some degree of it too. With extremes of these anatomies combined with sports or impact, one compartment can erode faster than the other.
Previous Ligament Damage & Arthritis
When you tear your ACL or MCL, there's stress placed on the knee at extreme range of motion. The bones may hit each other and cause a contusion, which can cause cartilage cells to die earlier than normal.
Without ACL reconstruction, the knee remains unstable, putting more pressure on the meniscus. Studies show that without ACL reconstruction, you're more prone to future meniscal tears, which can lead to earlier osteoarthritis.
Who Is a Candidate for Partial Knee Replacement?
Partial vs. Total Knee Replacement
A total knee involves resurfacing the end of the femur and top of the tibia on both sides—two large metallic components.
A partial (unicompartmental) knee replacement replaces only the medial or lateral compartment—both the femur and tibia surfaces, but only on one side of the knee.
The Ideal Partial Knee Candidate
Most unicompartmental candidates have varus (bow-legged) knees with erosion on the inside part of the knee. Key signs include:
Pain focused in one specific area ("right in here on the inside")
Feeling grinding on one side of the knee
On exam, the varus can be partially corrected toward neutral ("pseudo laxity")
X-ray shows degenerative changes isolated to the medial side
About 15-30% of patients with knee osteoarthritis are candidates for partial knee replacement.
Who Is NOT a Good Candidate?
Patients unsure where their pain is—they point everywhere
Those with a stiff deformity that can't be corrected with manipulation
Patients with arthritis in multiple compartments
Preparation & Nutrition
Prehabilitation
Everyone having knee surgery should have a good physiotherapy and exercise program pre-operatively—maintaining baseline step counts and activity levels while waiting for surgery.
Many patients have tried injections (hyaluronic acid, cortisone) over years to maintain function until they're ready for surgery.
The Role of Nutrition
Nutrition is key, especially for seniors. Sarcopenia (muscle loss) happens with hormonal changes. Exercise alone isn't enough—you have to feed your body the building blocks:
Protein — The most important nutrient; supplement with protein drinks if needed
Vitamin D — Essential for bone health
Creatine — Great evidence for building muscle mass, especially in the elderly
Surgical nutrition companies provide pre-surgery preparation solutions that can make a significant difference in recovery.
Surgical Technique
Anesthesia & Approach
Most patients receive a spinal anesthetic (frozen from the waist down) with sedation. An adductor canal block or catheter freezes a nerve that provides a lot of knee innervation for post-operative pain relief.
The incision is typically at the front of the knee, but many surgeons now use a subvastus (muscle-sparing) approach—preserving muscular and tendinous attachments for less pain and faster recovery, similar to the anterior hip approach.
Cemented vs. Uncemented Implants
Cemented implants use grout-like material to affix metal to bone—better for older patients or those with poor bone quality.
Uncemented implants have porosity that allows bone to grow into them. This latest generation does very well for patients who are heavier, younger, and more active.
Robotic-Assisted Surgery
How Robotics Changed Knee Surgery
Classically, surgeons were taught to make everyone's knee "perfectly straight," but people have different natural alignments. Robotics allows a personalized approach—not changing patients out of their comfort zone.
Robotic tracker pins are fixed to the femur and tibia. The robot maps alignment, cartilage, and bony landmarks. Surgical decisions are made based on what each patient needs for restoration of cartilage with bone and plastic.
Precision Beyond Human Capability
When trying to personalize implant positioning by a couple millimeters or a few degrees, the human eye can't detect or reproduce this accurately. Digital technology allows movement of implant pieces in 3D space before execution.
Robotics also quantifies soft tissue laxity—how much "give" there is in gaps. This balancing, once subjective, can now be measured and reproducibly taught.
Common Misconception: "The Robot Does the Surgery"
The surgeon doesn't go have coffee while R2-D2 operates. Current robotics are robotic-assisted—a robotic arm guides cuts and positioning while the surgeon performs the surgery.
Fully automated robots are coming—one company recently developed a system where AI and cameras could handle landmarking and algorithmic balancing. But we're still several years away from widespread autonomous robotic surgery.
Recovery & Outcomes
Partial vs. Total Knee Recovery
Even with robotics improving total knee outcomes, partial knees still recover faster, have less pain, and feel more natural. All the collateral ligaments are preserved rather than cut.
With total knee replacement, some patients describe the knee as not feeling like part of their body—clicks, clunks, and a mechanical feeling. This is much less common with partial knees.
Pain Management & Cryotherapy
Knee surgery is historically more painful than hip surgery—but the horror stories of two-week hospital stays are gone. Now, almost half of patients don't use any narcotics after knee replacement surgery.
Cryotherapy (cold compression devices like Game Ready) decreases the narcotic load needed, reduces inflammation and swelling. Not just a bag of peas—real cold and compression throughout the limb makes a significant difference.
Myths & Misconceptions
Myth: "The other compartment will need replacement soon"
In a well-picked candidate, this hopefully won't happen. It can happen if the partial knee is done incorrectly or overstuffed, putting pressure on the other side. Robotics and navigation reduce this risk by avoiding overtensioning.
Myth: "You'll have to give up your active lifestyle"
Look at Lindsey Vonn—she has a partial knee replacement and went to the Olympics. If your tendons, muscles, ligaments, and reaction time can handle it, you can work back up to high-level activity. Modern implant tolerances aren't the limiting factor anymore—it's about the patient's body.
Myth: "Partial knees don't last as long"
In high-volume centers, partial knees last 20+ years—comparable to total knee replacements. With robotics, better patient selection, and improved techniques, longevity should be just as good, if not better.
Think You Might Be a Candidate?
Book a consultation with Dr. Rodriguez to discuss whether partial knee replacement is right for your specific case. Virtual appointments available across Canada.
