Unicompartmental Knee Arthroplasty vs High Tibial Osteotomy (HTO)

Unicompartmental knee arthroplasty (UKA) and high tibial osteotomy (HTO) are both limb-preserving surgical options for medial compartment knee osteoarthritis, particularly in younger and middle-aged patients where total knee arthroplasty (TKA) is relatively contraindicated or undesirable due to higher activity demands, concerns about longevity, and the wish to preserve a more natural knee feel. Selecting the right procedure for the right patient requires understanding the indications, mechanics, outcomes, and failure modes of each.

• Both procedures address medial compartment overloading — the fundamental mechanical problem in varus knee OA
• HTO corrects the mechanical axis by realigning the tibia (or femur), offloading the medial compartment and transferring load to the healthier lateral compartment
• UKA replaces the damaged medial compartment surfaces while preserving cruciate ligaments and the lateral and patellofemoral compartments
• Neither procedure is universally superior — patient age, activity level, disease extent, anatomical factors, surgeon experience, and patient preference all determine which procedure is most appropriate
• Both can be followed by TKA if they fail — preserving this option is a key consideration in decision-making for young patients

HTO corrects varus malalignment by realigning the mechanical axis from the medial to the lateral compartment. The medial opening wedge osteotomy has largely replaced the lateral closing wedge technique in modern practice.

• Medial opening wedge HTO (MOWHTO): osteotomy of the proximal tibia opened on the medial side; fixed with locking plate (TomoFix, Puddu plate); gap filled with bone graft (autograft, allograft, or synthetic); preserves fibula; allows fine incremental correction; no lateral wound; most common technique in current practice
• Lateral closing wedge HTO (LCWHTO): wedge of bone removed from lateral tibial metaphysis; fibular osteotomy required; medial cortex used as hinge; historically more common; reliable healing at osteotomy site; less implant-dependent; shorter rehabilitation
• Distal femoral osteotomy (DFO): for valgus deformity with lateral compartment OA — corrects femoral valgus; medial closing or lateral opening wedge; used when origin of valgus is femoral rather than tibial
• Target correction: mechanical axis should pass through 62–65% of the tibial width (Fujisawa point) on post-operative long-leg alignment film — slightly lateral of centre; slight overcorrection to valgus reduces medial compartment load maximally
• Correction per mm of opening: approximately 1° per mm of opening; use Hernigou formula or preoperative planning software to calculate required opening

• Delayed union / non-union: reported in 1–5% of MOWHTO; risk increases with large corrections (>12 mm opening), smoking, obesity, and inadequate bone grafting; management with bone stimulator, supplementary grafting, or plate revision
• Lateral cortex fracture (hinge fracture): the most common intraoperative complication of MOWHTO — incomplete lateral cortex is the planned hinge; if the hinge fractures completely, fixation and stability are compromised; prevent by stopping the saw cut 5–10 mm from the lateral cortex and completing with an osteotome
• Patellar height change: MOWHTO increases tibial slope and may cause patella infera (Caton-Deschamps index decreases); can contribute to patellofemoral symptoms; LCWHTO typically associated with patella infera from proximal tibial shortening
• Posterior tibial slope change: MOWHTO increases tibial slope if posterior cortex is not carefully managed; excessive slope increases ACL tension and anterior tibial translation; relevant in ACL-deficient patients and PCL injuries
• Neurovascular: peroneal nerve at risk in lateral closing wedge osteotomy (traction on fibular osteotomy); compartment syndrome from swelling around osteotomy
• Infection: plate and hardware infection; deep infection requires hardware removal after osteotomy healing

• HTO + ACL reconstruction: simultaneous correction of varus malalignment and ACL insufficiency in young patients with both conditions; varus malalignment increases graft failure rates after isolated ACL reconstruction — combined procedure addresses both pathologies
• HTO + cartilage repair (ACI/MACI/microfracture): most powerful combination for young active patients with focal medial chondral lesions and varus malalignment; correcting the axis protects the chondral repair from continued overloading; excellent results reported from specialist centres
• HTO + meniscal allograft transplantation: for young patients with post-meniscectomy syndrome and varus malalignment; HTO corrects the axis while meniscal allograft restores medial compartment load distribution; technically demanding; evidence from specialist centres shows good medium-term outcomes
• UKA + ACL reconstruction: controversial; some evidence supports concurrent UKA and ACL reconstruction in appropriately selected patients with ACL deficiency and medial OA; more commonly ACL is reconstructed first and UKA performed if OA progresses

• The OAKS trial (UK, ongoing): multicentre RCT comparing UKA vs HTO for medial compartment OA in patients aged 45–70; designed to provide definitive comparative evidence; results eagerly awaited; current practice based largely on observational data, surgeon preference, and biomechanical rationale
• Failed HTO conversion to TKA: the pre-existing tibial deformity from osteotomy changes the tibial resection level and slope; hardware removal planned 6–12 weeks before TKA conversion to allow screw tracks to fill; tibial deformity may require additional corrective resection or custom instrumentation; outcomes of TKA after HTO are slightly inferior to primary TKA but generally satisfactory
• Double level osteotomy (DLO): for patients with varus deformity originating from both the tibia and femur — combined HTO + distal femoral osteotomy; corrects deformity at the true level of deformity rather than artificially at one level; technically demanding; used in complex deformity cases and planned corrections requiring >15° total correction
• Tibial slope in HTO: deliberate manipulation of tibial slope during HTO is used as an adjunct treatment — increasing slope in ACL deficiency reduces anterior tibial translation; decreasing slope in PCL deficiency or anterior compartment OA may be beneficial; slope changes must be planned pre-operatively

• HTO ideal patient: age <55–60, active, isolated medial OA, varus deformity, intact lateral compartment, no inflammatory arthritis
• Fujisawa point: mechanical axis to pass through 62–65% of tibial width post-HTO — slightly lateral overcorrection
• MOWHTO most common technique: medial opening wedge, TomoFix plate, bone graft; lateral cortex hinge — most common intraoperative complication is hinge fracture
• UKA preferred over HTO: older patient (>60), lower activity demand, intact ACL, minimal deformity, faster recovery desired
• HTO preferred over UKA: younger (<55), high-impact sport desired, ACL deficient (combine with ACLR), cartilage repair candidate, large varus deformity
• TKA conversion after HTO: more complex than after UKA — tibial deformity; hardware removal 6–12 weeks before TKA
• MOWHTO + MACI/ACI: powerful combination for young patients with focal chondral lesion + varus — correct axis to protect repair
• Lateral cortex hinge fracture: prevent by stopping saw 5–10 mm from lateral cortex; complete with osteotome
• ACL deficiency + varus OA: combined HTO + ACL reconstruction; isolated ACLR in varus knee has higher graft failure rate
• Double level osteotomy: for combined tibial and femoral origin of varus >15° total deformity