Knee Replacement:
TKA, UKA & PFA explained

Three procedures, one decision framework

The knee has three compartments: the medial (inner), lateral (outer), and patellofemoral (the joint between the kneecap and the front of the femur). Arthritis can affect one, two, or all three. The extent of involvement determines which operation — if any — makes sense.

Procedure	What is replaced	Typical indication	Hospital stay
TKA	All three compartments	Tricompartmental OA	2–4 weeks
UKA	One compartment (medial or lateral)	Single-compartment OA, intact ligaments	Within 2 weeks
PFA	Patellofemoral joint only	Isolated PF arthritis, intact tibial compartments	Within 2 weeks

Total Knee Arthroplasty (TKA)

TKA resurfaces all three compartments of the knee. The cartilage and a thin layer of bone are removed from the end of the femur, the top of the tibia, and the back of the patella, and replaced with metal and plastic components. The result is a smooth, pain-free joint surface.

Kinematic alignment — why it matters

For many years, TKA was performed using mechanical alignment: the implants were placed to achieve a perfectly straight leg (neutral mechanical axis), regardless of what the patient's native anatomy looked like. This was standardised and reproducible, but produced a "corrected" knee that didn't necessarily match how the individual's knee moved naturally.

Kinematic alignment (KA) takes a different approach. Rather than aiming for a neutral mechanical axis, KA aims to reproduce the patient's own constitutional knee anatomy — restoring the original joint line orientation and the native positions of the femoral condyles. The goal is a knee that moves and feels more like the patient's own joint.

It is important to note that KA is a surgical philosophy, not a single technique — there are multiple ways to implement it (restricted KA, functional KA, etc.), and the literature is still evolving. My approach is to use pre-operative planning based on each patient's anatomy and to choose the alignment strategy most appropriate for their individual knee geometry.

The midvastus approach

The standard approach to TKA uses a medial parapatellar arthrotomy — a vertical incision through the quadriceps tendon alongside the patella. The midvastus approach modifies this by splitting the vastus medialis oblique (VMO) muscle fibres rather than cutting through the tendon junction. This preserves more of the extensor mechanism integrity.

The clinical rationale is improved quadriceps function in the early post-operative period. A review of the evidence (Tzatzairis et al., Journal of Orthopaedics, 2018) found that minimally invasive approaches including the midvastus are associated with advantages in early recovery, including earlier straight-leg raise, reduced pain, and shorter hospital stay in some studies — with the caveat that these benefits must be weighed against exposure quality to avoid technical error.

I use the midvastus approach for primary TKA in most patients where adequate exposure allows. For revision or complex primary cases, standard medial parapatellar exposure is used when necessary.

Unicompartmental Knee Arthroplasty (UKA)

UKA replaces only the damaged compartment — most commonly the medial compartment. The advantages over TKA include a smaller incision, less bone removal, faster recovery, and preserved proprioception (the knee's ability to sense position). The feeling is often described as more "natural" because the intact compartments and ligaments are preserved.

UKA is appropriate for patients with single-compartment arthritis, intact anterior cruciate ligament, and a correctable deformity. Roughly 20–25% of patients presenting with knee OA may be candidates. Careful patient selection is critical — placing a UKA in a knee with diffuse disease leads to early failure.

Patellofemoral Arthroplasty (PFA)

PFA replaces the patellofemoral joint — the joint between the kneecap (patella) and the front groove of the femur (trochlea). It is indicated for isolated patellofemoral osteoarthritis: pain specifically around the kneecap, particularly with stairs, squatting, and prolonged sitting, in the absence of significant tibial compartment disease.

PFA is less common than UKA but is an important option for patients — often younger, active individuals — whose arthritis is genuinely confined to the front of the knee. Like UKA, patient selection is key: if tibial compartment disease is overlooked, the operation will fail to relieve all the patient's pain.

Recovery after knee replacement

TKA recovery takes longer than hip replacement recovery. Expect 2–4 weeks in hospital, and 3–6 months before the knee feels fully functional. Physiotherapy is essential and begins from day one. UKA and PFA recoveries are generally faster, with most patients achieving good function within 6–8 weeks.

If you have a concurrent ligament or meniscus injury, these can often be addressed at the same time. See the articles on ACL reconstruction and meniscus surgery for more detail.