The short answer: 1 to 2 hours in theatre

For both total knee replacement (TKR) and partial knee replacement (UKR), theatre time on a typical case runs from 90 to 120 minutes — roughly one to two hours from the first incision to wound closure. That figure holds across NHS guidance and specialist orthopaedic centres, and it applies whether the surgeon is resurfacing the entire joint or only a single compartment.

It is worth being clear about what those 90 to 120 minutes cover. They represent operative time: the period when the surgeon is actively working on the knee. They do not include the time needed to administer anaesthesia beforehand, or the recovery-room period afterwards while the anaesthetic wears off. Patients should therefore expect to be in the surgical unit for considerably longer than two hours on the day of their procedure.

For uncomplicated cases, this operative window is well established and predictable — which helps patients plan sensibly ahead of admission. Certain factors can extend it beyond the standard range, and these are covered in the sections that follow.

What happens during a total knee replacement — and why it takes that long

Total knee replacement is indicated when osteoarthritis has affected the whole joint or multiple compartments, leaving insufficient healthy cartilage to preserve. The procedure follows a fixed sequence — and it is that sequence, rather than any single step, that accounts for the 90 to 120 minutes of theatre time.

Once anaesthesia is established, the surgeon makes an anterior midline incision over the knee and carefully moves the patella (kneecap) to one side to expose the joint surfaces beneath. Damaged bone is then resected from the lower end of the femur and the upper end of the tibia using precision cutting guides; achieving the correct angles at this stage is critical, as they determine how the replacement joint will sit and move.

With the bone ends prepared, the metal femoral and tibial components are positioned and secured — either cemented with surgical bone cement or press-fitted, depending on the patient's bone quality and the surgical plan. A polyethylene bearing surface is then seated between them to replicate the cushioning function of natural cartilage. If the underside of the patella is worn, it may be resurfaced as a separate step before the wound is finally closed in layers using sutures or clips.

Each stage must be completed in sequence and to fine tolerances. Bone cuts cannot be revisited quickly if alignment is off, and implant fixation requires adequate setting time. This is why even a straightforward TKR cannot meaningfully be compressed below the standard operative window.

Partial knee replacement (UKR) — a narrower procedure, similar timeframe

Partial knee replacement addresses a different clinical picture: arthritis confined to one compartment — most often the medial (inner) side — while the remaining cartilage and ligaments are structurally intact. Through a smaller anterior incision, the surgeon removes and resurfaces only the damaged bone surfaces in that single compartment, leaving the healthy side of the joint, the cruciate ligaments, and the surrounding soft tissue entirely undisturbed.

That narrower footprint does not, however, translate into a dramatically shorter operation. Theatre time for UKR falls within or just below the standard 90-to-120-minute range — not the half-hour shortcut that some patients expect from a "partial" procedure. One reason is that UKR demands more intensive pre-operative planning than TKR: custom three-dimensional templating and precision instrumentation are required to ensure the new component fits accurately alongside the preserved compartments. Errors in positioning carry consequences for the healthy side of the joint, so the planning burden is genuinely higher even if the operative footprint is smaller.

Head-to-head figures comparing mean operative minutes between UKR and TKR are not firmly established in published literature. In practice, many UKR cases tend to complete in the lower portion of that 90-to-120-minute window, while straightforward TKRs more often reach the upper end — though individual variation means neither figure should be taken as a personal prediction.

The most important distinction between the two procedures is eligibility rather than duration. UKR is appropriate only when osteoarthritis is genuinely confined to one compartment and both cruciate ligaments remain intact — criteria that apply to a smaller proportion of people presenting with knee OA than those suitable for TKR. Imaging and a consultant assessment are needed to determine which approach suits a specific joint.

Anaesthesia: what patients experience before and during surgery

Before the surgeon makes a single incision, an anaesthetist will have agreed with the patient on the most appropriate form of pain control. For most knee replacements — whether total or partial — the choice lies between two main routes: a general anaesthetic, which renders the patient fully unconscious, or a spinal (regional) block, which removes all sensation from the waist downward while the patient remains awake and comfortable.

In practice, many contemporary units use a combination: a spinal block supplemented with peripheral nerve blocks to target specific pain pathways around the knee, with light sedation added where the patient prefers not to be aware of the theatre environment. This multimodal approach reduces systemic drug exposure and tends to leave patients more alert in the immediate hours after surgery — a practical benefit, since early mobilisation is central to modern recovery protocols for both TKR and UKR.

Anaesthetic induction and positioning add time beyond the operative window described above. Patients should expect to be in the surgical unit for several hours in total, which includes the recovery period while sensation and mobility gradually return.

The exact anaesthetic plan is agreed individually between the patient and the anaesthetist, taking into account personal health factors, existing medications, and patient preference.

Factors that can extend time in theatre

Several factors can push a case beyond the 90-to-120-minute baseline, and understanding them helps patients ask better questions before their operation.

Joint complexity is one of the most consistent determinants of duration. Significant varus or valgus deformity, severe bone loss, or a joint that has undergone previous surgery all extend the resection and soft-tissue balancing phases. A severely malaligned knee requires careful incremental adjustment that a straightforward arthritic joint does not — and there is no shortcut through that process.

Robotic-assisted surgery — such as Mako-assisted TKR or UKR — introduces additional intra-operative steps: the joint must be registered to a pre-operative CT model before bone cutting begins, and the robotic arm verifies implant positioning in real time throughout placement. These registration steps add to theatre time, though the corresponding benefit is greater precision in component alignment — a relevant trade-off rather than a drawback in itself.

Bilateral knee replacement — replacing both knees in one session — is performed in selected patients and, by definition, adds roughly another full procedure's worth of time. It is less commonly undertaken than staged bilateral replacement, and the decision involves careful anaesthetic planning.

Increased soft-tissue depth in patients with higher BMI can extend the exposure phase: gaining and maintaining a clear operative field requires additional time proportional to tissue thickness.

Finally, surgeon experience and team familiarity influence efficiency throughout. A high-volume knee surgeon working within a familiar theatre team will typically complete the same procedure more quickly than an equivalent case in a less practised setting — a factor worth considering when choosing where to have the operation.

Your total time in the surgical unit — and what comes after

The 90-to-120-minute operative window is only one part of the patient's day in the surgical unit. Pre-operative preparation — consent checks, cannulation, anaesthetic induction, and positioning — precedes the first incision, and time in the recovery room follows the last suture. Taken together, patients should plan for several hours in the unit from arrival to ward transfer, not simply the operative figure quoted at their consultation.

Once the procedure is complete, patients move to a recovery room where nursing staff monitor vital signs and pain levels until the anaesthetic has sufficiently worn off. For those who received a spinal block, this includes waiting for sensation and motor control to return to the lower limbs before transfer to the ward is considered safe.

For a straightforward TKR or UKR, hospital stay is typically one night. Physiotherapy begins early — most units aim to get patients standing and taking supervised steps within the first day post-surgery, which supports circulation and reduces the risk of complications.

The longer-term picture differs between the two procedures. Full functional recovery from TKR is a gradual process that can take up to one year, with progressive milestones in strength, range of movement, and confidence on the leg across that period. UKR's smaller surgical footprint — less bone resection, less soft-tissue disruption — is associated with a shorter functional recovery window, though the exact timeline varies between individuals and is best discussed in the context of a specific clinical assessment.

Expected recovery is one of the most common questions patients bring to their first consultation — and one where honest, case-specific answers matter more than general estimates.

1. [1] Unicompartmental Knee Arthroplasty – Wikipedia. https://en.wikipedia.org/?curid=16991704 https://en.wikipedia.org/?curid=16991704