Gonarthrosis (Knee Osteoarthritis)

Introduction

Gonarthrosis — osteoarthritis of the knee — is the commonest joint disease in adults and one of the leading causes of chronic disability worldwide. The condition affects all three compartments of the knee (medial tibiofemoral, lateral tibiofemoral, patellofemoral) in varying patterns and combinations, producing progressive pain, stiffness, deformity, and functional limitation. Treatment ranges from lifestyle modification and pharmacological therapy in early disease through joint-preserving surgical procedures (high tibial osteotomy, unicompartmental arthroplasty) for selected patients, to total knee arthroplasty for end-stage disease. This chapter synthesizes content from Apley & Solomon’s, Miller’s Review, Dutton’s Orthopaedic Examination, and Orthopaedic Surgical Approaches.

Epidemiology and Risk Factors

Symptomatic knee osteoarthritis affects approximately 10-15% of adults over age 60, with the prevalence rising substantially with age and reaching nearly 30% in those over 80. The lifetime risk of symptomatic knee osteoarthritis is approximately 45% — higher than for the hip. Women are affected more often than men. The principal risk factors are: age, obesity (the strongest modifiable risk factor for knee OA, with each unit increase in BMI substantially raising risk), female sex, prior knee injury (ACL injury, meniscal injury, intra- articular fracture; the development of OA after meniscectomy is well-documented and has driven the modern preference for meniscal repair over resection), occupational and recreational exposure to heavy loading and squatting, malalignment (varus alignment predisposes to medial compartment disease, valgus alignment to lateral compartment disease), prior infection, hemophilia, inflammatory arthritis, and genetic factors.

Pathophysiology

The pathological process of knee osteoarthritis follows the general principles of osteoarthritis described in the preceding chapter on hip OA, with the additional feature that the knee comprises three articulating compartments with different mechanical demands and patterns of involvement. Medial compartment osteoarthritis is the most common pattern in many populations, with progressive varus deformity reflecting the asymmetric cartilage loss. Lateral compartment disease is less common and typically presents with valgus deformity. Patellofemoral disease may occur in isolation (often associated with patellar maltracking, patella alta, or trochlear dysplasia) or as part of multi- compartment disease.

Clinical Features

Presentation is with progressive knee pain typically localized to the affected compartment(s), often described as deep, aching, and exacerbated by weight-bearing and use. Mechanical symptoms — catching, locking, giving way — may indicate meniscal pathology or loose bodies. Stiffness, particularly after periods of rest (“gelling”), is

characteristic and improves with gentle motion. Functional limitations include difficulty with stairs (descending is typically worse than ascending), difficulty rising from chairs, reduced walking distance, and limp. Examination findings progress with disease severity: antalgic gait; varus or valgus deformity on weight-bearing (often more pronounced than supine); joint-line tenderness localized to the affected compartment; effusion (typically modest); reduced range of motion with flexion contracture in advanced disease; crepitus on motion; quadriceps wasting; positive McMurray test (less reliable in established OA); and positive grind test for patellofemoral disease.

Imaging

Standardized weight-bearing radiographs are essential, since the joint-space narrowing of knee OA is reliably demonstrated only with axial loading. The standard views include: AP weight-bearing view with knees in extension (or with knees in 30-45° flexion — the Rosenberg view, which is more sensitive for posterior compartment disease); lateral view; sunrise (Merchant) view for the patellofemoral joint; and standing long-leg alignment view for measurement of mechanical and anatomical axes. The Kellgren-Lawrence classification, applied to the knee, has the same grades as for the hip (0-4); the Ahlbäck classification is an alternative knee-specific system with five grades emphasizing joint-space narrowing and subchondral changes. MRI provides additional information when needed: assessment of meniscal pathology, ligament injury, subchondral bone marrow lesions (which have prognostic significance for symptom progression and surgical outcome), early cartilage damage, and alternative diagnoses such as spontaneous osteonecrosis of the knee (SONK), pigmented villonodular synovitis, and intra-articular tumors. MRI is not routinely required for the diagnosis of knee OA but is valuable in atypical presentations.

Non-Operative Management

The non-operative management of knee OA combines patient education, lifestyle modification, pharmacological treatment, and selective injection therapies, following the AAOS, ACR, and NICE evidence-based guidelines. Weight loss is the most powerful single non-operative intervention for the obese patient with knee OA. The mechanical effect (each kilogram of weight loss reduces the load on the knee by approximately 3-4 kg with each step) is supplemented by reduction of systemic inflammation. Weight loss of 5-10% produces meaningful improvement in pain and function in most patients. Exercise — combining aerobic exercise, quadriceps strengthening, and range-of-motion exercises — is among the most effective and most underutilized treatments for knee OA. Quadriceps strengthening in particular has a robust evidence base for symptom relief. Activity modification addresses avoidance of high-impact and aggravating activities, with substitution by lower-impact alternatives.

Pharmacological treatment: Paracetamol for mild pain, NSAIDs (oral or topical) for moderate pain, opioids reserved for severe cases unresponsive to other measures. Intra-articular injections: Corticosteroid injection produces short-term symptomatic relief in most patients but with limited duration of effect (typically 4-12 weeks) and concerns about cumulative cartilage damage with repeated injections (the FDA has warned about cartilage loss with frequent steroid injections). Hyaluronic acid (viscosupplementation) injections produce modest benefit in some patients, with current guidelines giving qualified recommendations. Platelet-rich plasma (PRP) injection is under continued investigation, with some evidence of benefit in early to moderate OA but with substantial heterogeneity in preparations and protocols. Stem cell injections remain investigational. Bracing and orthotics: Unloader braces for medial compartment OA in younger active patients with isolated medial disease can produce meaningful symptom relief. Lateral wedge insoles for medial compartment OA have a smaller and less consistent benefit. Walking aids: A cane in the contralateral hand reduces the joint reaction force on the affected knee by 20-40%.

Joint-Preserving Surgery

High Tibial Osteotomy High tibial osteotomy (HTO) is the classical joint-preserving operation for medial compartment knee osteoarthritis with varus deformity in the younger active patient. The principle is to realign the mechanical axis of the lower limb so that the load is transferred from the worn medial compartment to the relatively preserved lateral compartment, allowing the patient to continue with reasonable function for many years before requiring knee replacement. The classical patient for HTO is: young to middle-aged (typically 40-60 years); active and unwilling to accept the activity restrictions of knee arthroplasty; medial compartment OA with preserved lateral compartment and reasonably preserved patellofemoral compartment; varus alignment; good range of motion (typically full extension and flexion to at least 90°); and good ligamentous stability. Contraindications include: bicompartmental or tricompartmental disease; severe valgus deformity (relative contraindication for HTO, which produces medial-to-lateral transfer); inflammatory arthritis; obesity (relative contraindication — modest obesity is acceptable but severe obesity is associated with worse outcomes); flexion contracture or limited motion; and unrealistic expectations. The two principal techniques are: opening-wedge HTO (medial proximal tibial cut with gradual opening of the wedge, fixed with a plate) — favored in modern practice because of better control of correction, no fibular osteotomy needed (the fibula is “freed” by the opening), and no bone removal; and closing-wedge HTO (lateral proximal tibial closing wedge with removal of a triangular bone fragment) — the classical technique, with the

advantage of immediate stability without need for bone graft but with the disadvantages of fibular osteotomy (with associated peroneal nerve risk) and shortening of the limb. The target alignment for HTO in medial compartment OA is typically slight valgus mechanical axis (3-5° valgus), which transfers load to the lateral compartment and “rests” the medial compartment. The procedure provides good symptom relief in 80-90% of well- selected patients at 5 years, dropping to 60-70% at 10 years, with progressive failure typically requiring conversion to TKA in the subsequent decade. Distal Femoral Osteotomy Distal femoral osteotomy is the analogous procedure for lateral compartment osteoarthritis with valgus deformity. The principle is to realign the mechanical axis to transfer load from the worn lateral compartment to the relatively preserved medial compartment. The procedure is less commonly performed than HTO because lateral compartment disease is less common and because valgus deformities can also be addressed by HTO with appropriate correction. The technique typically involves a lateral closing-wedge or medial opening-wedge distal femoral osteotomy. Cartilage Restoration Procedures For focal full-thickness cartilage defects in the otherwise preserved knee, several cartilage restoration procedures are used: debridement (smoothing of damaged cartilage surfaces, of limited durable benefit); microfracture (penetration of subchondral bone to stimulate fibrocartilage formation, useful for small defects); autologous chondrocyte implantation (ACI) and matrix-assisted ACI (a two-stage procedure of cartilage harvest, ex vivo expansion, and re-implantation, used for larger defects); osteochondral autograft transplantation (mosaicplasty, OATS) using cylindrical osteochondral plugs from non- weight-bearing areas; and osteochondral allograft transplantation for large or complex defects. These procedures are generally reserved for focal cartilage defects in younger patients without established osteoarthritis; in established OA, the surrounding articular damage and altered joint mechanics produce poor outcomes.

Unicompartmental Knee Arthroplasty (UKA)

Unicompartmental knee arthroplasty replaces only the affected compartment(s) of the knee, preserving the unaffected compartments and the cruciate ligaments. The medial UKA is the most common application; lateral and patellofemoral UKA are also performed. The classical indications for medial UKA (the Kozinn and Scott criteria) include: isolated medial compartment osteoarthritis with full-thickness cartilage loss; preserved lateral compartment and patellofemoral compartment (with full-thickness cartilage); intact anterior cruciate ligament; correctable varus deformity (passively correctable to at least neutral); flexion contracture less than 5°; flexion range of motion to at least 90°; and age over 60 years with weight under 82 kg (the original criteria; modern practice has loosened these limits). Contraindications include inflammatory arthritis, advanced disease in other compartments, ACL deficiency, severe deformity, and substantial flexion contracture.

Modern UKA produces excellent functional outcomes — frequently superior to TKA for selected patients — with preservation of native knee kinematics, faster recovery, smaller incision, less blood loss, and easier rehabilitation. The disadvantages include the more demanding implantation technique (with higher early revision rates in low-volume surgeons), the potential for progression of disease in the unreplaced compartments requiring later conversion to TKA, and the more stringent indications. Long-term survivorship has improved over recent decades and modern series report 10-year survival of 90% or higher in centers with appropriate volume and patient selection.

Total Knee Arthroplasty (TKA)

Indications TKA is indicated for end-stage tri-compartmental or bi-compartmental knee osteoarthritis producing symptoms unresponsive to non-operative management, in patients with appropriate functional demands, comorbidities, and life expectancy. The procedure is also performed for advanced rheumatoid arthritis, post-traumatic arthritis, and the rare cases of post-infectious arthritis after appropriate decontamination.

Implant Design Modern TKA implants comprise four principal components: a femoral component (cobalt- chromium, with various designs); a tibial component (typically titanium baseplate with modular polyethylene insert); a polyethylene insert (with options including conventional, highly cross-linked, and increasingly vitamin E-stabilized polyethylene; cruciate-retaining or cruciate-substituting designs; fixed-bearing or mobile-bearing); and an optional patellar component (resurfacing of the patella with a polyethylene button is performed routinely by many surgeons and selectively by others). The principal design decisions include: Cruciate-retaining (CR) vs cruciate-substituting (CS, posterior-stabilized, PS) designs: CR designs preserve the posterior cruciate ligament; PS designs sacrifice the PCL and use a cam-and-post mechanism to provide AP stability. Both designs have produced good long-term outcomes; the choice is largely surgeon preference. Fixed-bearing vs mobile-bearing designs: Fixed-bearing implants have a polyethylene insert locked to the tibial baseplate; mobile-bearing implants allow the polyethylene insert to rotate or translate on the baseplate, theoretically reducing polyethylene stress. Clinical outcomes are similar between the two; mobile-bearing implants have specific complications (insert dislocation) but theoretical advantages in younger patients. Patellar resurfacing: Most authors advocate routine patellar resurfacing in modern TKA, with the patella replaced with a polyethylene button. Selective resurfacing (resurfacing only when the patellar cartilage is severely damaged) is favored by some surgeons.

Surgical Technique The principles of modern TKA include: appropriate preoperative templating; the medial parapatellar arthrotomy (the most common approach) or subvastus or midvastus approach (more anatomically sparing alternatives); careful bone cuts using either conventional jigs, computer navigation, or robotic systems; balancing of the flexion and extension gaps with appropriate ligament releases; trial reduction with assessment of alignment, stability, and patellar tracking; cementation (or uncemented fixation in selected patients) of components; and meticulous wound closure. Alignment philosophy has evolved over recent years from the historical “mechanical alignment” target (neutral mechanical axis, perpendicular to the floor) to “kinematic alignment” (matching the patient’s pre-arthritic anatomy) and various hybrid approaches. The optimal alignment philosophy remains debated, with both approaches producing good outcomes in different series. Robotic-assisted TKA, computer navigation, and patient-specific instrumentation have grown in popularity, with several large series demonstrating improved precision of component placement; whether this precision translates to better long-term clinical outcomes is the subject of ongoing investigation. Outcomes of TKA Modern TKA produces excellent outcomes for the great majority of patients, with 10-year survivorship of approximately 95% and 20-year survivorship of approximately 80-85% in large registry data. Patient-reported outcomes show substantial improvement in pain and function, although 15-20% of patients report some degree of persistent dissatisfaction even after technically successful surgery — the so-called “20% dissatisfaction rate” that has driven much research into preoperative selection, expectation management, and surgical technique refinements. Complications of TKA The principal complications of TKA include: Infection (PJI): Rate of 1-2% for primary TKA; management is similar to that for hip PJI as discussed in the chapter on bone and joint infections. Stiffness: A particular problem in TKA. Flexion contracture, limited flexion, or both produce functional disability. Manipulation under anesthesia is often successful in the early postoperative period (6-12 weeks); later stiffness may require arthroscopic adhesion release or revision surgery. Patellar problems: Patellofemoral pain, maltracking, instability, fracture, and avascular necrosis are all recognized; modern attention to component rotation has reduced rates. Periprosthetic fracture: Distal femoral fractures above a TKA femoral component are the most common, often in osteoporotic bone. Treatment is by retrograde IM nailing, locking

plate fixation, or revision arthroplasty depending on fracture morphology and implant fixation. Aseptic loosening: Once common, now uncommon with modern implants and techniques. Polyethylene wear and osteolysis: Substantially reduced with highly cross-linked polyethylene. Thromboembolic disease: Higher risk than for hip replacement, with prophylaxis similar to that for hip arthroplasty. Neurovascular injury: Peroneal nerve palsy (particularly after correction of severe valgus deformity); popliteal artery injury (rare but catastrophic).

Special Considerations

Younger Patients Knee arthroplasty in younger patients (<60 years) has higher revision rates than in older patients, reflecting the higher activity demands and the cumulative effects of bearing wear. For younger active patients with appropriate disease patterns, joint-preserving surgery (HTO, UKA) may offer durable function with delay of the eventual TKA.

Inflammatory Arthritis TKA in rheumatoid arthritis and other inflammatory arthropathies follows the principles of OA TKA with attention to systemic medical management, perioperative coordination with the rheumatologist, careful management of biological therapy timing perioperatively (current guidelines recommend hold of most biologics 1-2 dosing intervals before surgery), and management of associated soft-tissue deformities (severe valgus, flexion contracture, etc.).

Post-Traumatic Arthritis Post-traumatic knee arthritis after intra-articular fracture or ligament injury often presents in younger patients than primary OA. The technical demands are increased by prior surgical incisions, hardware, deformity, and ligament insufficiency. Specific challenges include the management of leg-length discrepancy from prior physeal injury, the management of hardware (often requiring removal in a staged procedure), and the use of revision components in cases with substantial bone or ligament deficiency. Revision TKA Revision TKA addresses failed primary TKA from infection, loosening, stiffness, instability, periprosthetic fracture, or polyethylene wear and osteolysis. The technical demands are substantially greater, with management of bone defects (Anderson Orthopaedic Research Institute — AORI — classification of distal femoral and proximal tibial defects), the use of revision components with augments and stems for additional fixation, and addressing the underlying cause of failure to prevent recurrence.

Summary and Take-Home Points

Gonarthrosis is the commonest joint disease in adults and a leading cause of chronic disability. The condition affects the medial, lateral, and patellofemoral compartments in various patterns and combinations. Clinical assessment combining history, physical examination, and standardized weight-bearing radiographs establishes the diagnosis and stages the disease. Non-operative management combining weight loss (the most powerful single intervention in the obese patient), exercise, activity modification, pharmacological treatment, and selective intra-articular injections provides the foundation of care. Joint- preserving surgery — high tibial osteotomy for medial compartment disease with varus, distal femoral osteotomy for lateral compartment disease with valgus, and unicompartmental knee arthroplasty for isolated single-compartment disease — provides durable function for selected younger patients. Total knee arthroplasty is the standard for end-stage tri-compartmental disease and produces excellent outcomes with 10-year survivorship of approximately 95% and substantial improvement in pain, function, and quality of life. The principal complications of infection, stiffness, periprosthetic fracture, and aseptic loosening are managed by careful surgical technique and appropriate implant selection. The persistent 15-20% dissatisfaction rate after technically successful TKA has driven ongoing research into patient selection, expectation management, and refinement of surgical technique.