Orthopaedic Surgery - Knee Replacement

Basics

Total knee arthroplasty is a highly effective surgical treatment for advanced, disabling arthritis of the knee.

The operation resurfaces the damaged articular surfaces of the femur and tibia with metallic components separated by a polyethylene bearing surface. Depending on the procedure, the patellar surface may also be resurfaced.

The prosthetic components are shaped to reproduce normal knee geometry as closely as possible while restoring alignment, stability, and motion.

Classification by Compartment

Knee arthritis can primarily involve one or more compartments.

The major compartments are:

Medial tibiofemoral compartment

Lateral tibiofemoral compartment

Patellofemoral compartment

The distribution of arthritis helps determine whether a patient is a candidate for partial or total knee replacement.

Epidemiology

Knee arthroplasty is performed commonly and its use has continued to increase.

Historically, approximately 61% of total knee replacements have been performed in women.

Older U.S. data recorded approximately 418,000 procedures in 2003, and the annual number has increased substantially since that time.

The average age at surgery in older series was approximately 67 years, although arthroplasty is now performed across a broader age range.

Risk Factors

Factors that contribute to knee degeneration severe enough to require replacement include:

Previous trauma, meniscectomy, obesity, malalignment, inflammatory disease, and chronic joint overload.

Etiology

Osteoarthritis

Primary osteoarthritis is the most common indication for knee replacement.

In many cases it is idiopathic, although age, obesity, genetics, previous injury, and joint mechanics may contribute.

Post-Traumatic Arthritis

Post-traumatic arthritis may develop after:

Sports injuries, fractures, ligament injuries, meniscal loss, falls, or motor vehicle trauma.

Previous injury may produce altered alignment or instability that accelerates cartilage degeneration.

Inflammatory Arthritis

Inflammatory joint disease may also lead to severe cartilage destruction.

Examples include:

Rheumatoid arthritis, gout, pseudogout, and other inflammatory arthropathies.

Associated Conditions

Patients with advanced knee arthritis often have degenerative disease elsewhere.

Common associated conditions include:

Hip arthritis, lumbar spinal stenosis, and foot or ankle arthritis.

In patients with rheumatoid arthritis, foot and ankle deformity may be particularly severe and may influence gait and rehabilitation.

Diagnosis

Signs and Symptoms

Common signs include:

Joint effusion, joint-line tenderness, deformity, reduced motion, and an antalgic gait.

Deformity

Osteoarthritis commonly produces a varus deformity, particularly when the medial compartment is most affected.

Valgus deformity may occur with lateral compartment disease and has historically been associated with inflammatory arthritis such as rheumatoid arthritis.

Symptoms

Typical complaints include:

Pain, start-up pain, swelling, catching, instability or giving way, stiffness, and progressive limitation of daily activities.

History

The patient often reports pain that has progressively worsened despite appropriate nonoperative treatment.

Important historical features include:

Duration of symptoms, walking tolerance, night pain, stair difficulty, need for assistive devices, previous knee surgery, instability, and response to medication or injections.

Persistent pain that is no longer adequately relieved by conservative treatment supports consideration of arthroplasty when imaging findings are compatible.

Physical Examination

The knee should be examined systematically for signs of arthritis.

Effusion

An effusion may reflect:

Synovitis, degenerative disease, inflammatory arthritis, or another intra-articular process.

Joint-Line Tenderness

Medial or lateral joint-line tenderness may result from:

Compartmental arthritis or meniscal pathology.

Other Tenderness

The examiner should assess for tenderness over:

The patellofemoral joint, collateral ligaments, pes anserine region, patellar tendon, and surrounding soft tissues.

Range of Motion

Document:

Flexion, extension, flexion contracture, extension lag, and pain through the arc of motion.

Marked loss of motion may affect both operative planning and postoperative rehabilitation.

Gait

Assess for:

Antalgic gait, varus or valgus thrust, flexion contracture, instability, and use of walking aids.

Laboratory Tests

Routine laboratory testing does not diagnose osteoarthritis.

Testing is directed toward alternative diagnoses or preoperative assessment.

Suspected Infection

If infection is a concern, useful studies include:

C-reactive protein and ESR.

Further investigation may require aspiration.

Suspected Gout

Serum uric acid may be obtained, although the diagnosis of gout is established more reliably through synovial fluid crystal analysis.

Imaging

Plain Radiographs

Weight-bearing radiographs are the first-line imaging study for knee arthritis.

Standing AP or PA View

A standing view can demonstrate subtle or advanced loss of joint-space width and allows comparison of medial and lateral compartments under physiologic load.

Lateral View

The lateral radiograph helps assess:

Patellofemoral degeneration, posterior osteophytes, alignment, and flexion deformity.

Patellofemoral View

A tangential or skyline view is useful for assessing:

Patellar alignment, trochlear disease, and patellofemoral joint-space loss.

MRI

MRI is not routinely required before knee replacement when the diagnosis of advanced osteoarthritis is clear on weight-bearing radiographs.

It may be useful in selected cases to evaluate:

Meniscal pathology, synovial proliferative disorders such as PVNS, osteonecrosis, occult tumor, or focal cartilage injury.

Arthrocentesis

Joint aspiration can be extremely helpful when the diagnosis is uncertain.

It may help evaluate for:

Septic arthritis, gout, or pseudogout.

Synovial fluid may be sent for:

Cell count, differential, Gram stain, culture, and crystal analysis.

Pathophysiology

The common feature of end-stage arthritis is progressive loss of articular cartilage.

Proteoglycans and normal cartilage matrix are lost, causing thinning and eventual exposure of subchondral bone.

As cartilage disappears, patients may develop:

Painful bone-on-bone contact, deformity, osteophytes, sclerosis, cyst formation, and progressive loss of motion.

Differential Diagnosis

Conditions that may mimic or coexist with knee arthritis include:

Septic arthritis, osteomyelitis, patellofemoral pain or instability, meniscal tears, crystal arthropathy, and tumor.

Treatment

General Principles

Nonoperative treatment should generally be attempted before elective knee replacement.

Options include:

NSAIDs, acetaminophen, weight reduction when appropriate, walking aids, braces, exercise-based physical therapy, activity modification, and intra-articular injections.

Younger Patients

In younger patients, especially those under approximately 50 years, joint-preserving procedures may be considered when disease is localized and anatomy is suitable.

These may include:

Realignment osteotomy or selected arthroscopic procedures for focal mechanical pathology.

The goal is to delay arthroplasty when possible without leaving the patient severely disabled.

Decision for Surgery

Knee replacement is considered when:

Pain and functional limitation are substantial, radiographs demonstrate significant joint degeneration, and appropriate nonoperative treatment has failed.

The patient should receive a detailed discussion of:

Expected benefits, limitations, rehabilitation, implant longevity, and potential complications.

Preoperative Preparation

Patients undergo a complete medical evaluation before surgery.

Important issues include:

Cardiovascular status, pulmonary disease, anemia, infection risk, anticoagulation, diabetes control, and nutritional status.

Routine preoperative autologous blood donation is now uncommon.

Anticoagulants and certain medications may need to be adjusted before surgery according to individual risk and perioperative guidance.

Physical Therapy

Physical therapy is a central component of recovery.

Range of Motion

A common early goal is to regain approximately:

0–110° of knee motion within the first several weeks, although individual recovery varies.

Strengthening

Rehabilitation emphasizes:

Quadriceps strength, hamstring strength, gait training, balance, transfers, and functional independence.

Weight Bearing

Most modern total knee replacement protocols allow early full weight bearing as tolerated when medically and surgically appropriate.

Medication

Postoperative Analgesia

Effective pain control is necessary to permit early mobilization and participation in therapy.

Modern care usually relies on a multimodal analgesic strategy, which may include:

Acetaminophen, NSAIDs when appropriate, regional anesthesia, periarticular injections, and limited opioid use for breakthrough pain.

Long-term opioid use is generally avoided.

Surgery

Knee replacement may be either:

Partial or total.

Unicompartmental Knee Replacement

A unicompartmental or partial knee replacement resurfaces only one diseased tibiofemoral compartment.

Indications

It is appropriate only when arthritis is isolated to a single compartment and the remaining knee structures are suitable.

Important considerations include:

Intact or functionally adequate ligaments, correctable deformity, preserved opposite compartment cartilage, and appropriate patient selection.

Compartments

Unicompartmental replacement may be performed for:

Medial or lateral compartment arthritis.

Patellofemoral Arthroplasty

Isolated patellofemoral replacement is another form of partial arthroplasty used in selected patients with disease confined to the patellofemoral compartment.

Total Knee Replacement

Total knee arthroplasty resurfaces the distal femur and proximal tibia and may also resurface the patella.

Surgical Exposure

A midline skin incision with a standard arthrotomy is commonly used.

Bone Preparation

Specialized guides or navigation systems are used to make femoral and tibial bone cuts and establish:

Alignment, component rotation, joint-line position, ligament balance, and flexion-extension gaps.

Component Fixation

Components may be:

Cemented, uncemented, or hybrid.

Cemented fixation remains common, while modern porous-coated components may permit biologic ingrowth.

Patellar Resurfacing

The patellar articular surface may be resurfaced depending on:

Surgeon preference, patellar cartilage condition, implant system, and patient factors.

Bearing Surface

The femoral component articulates against highly cross-linked polyethylene on the tibial side.

This metal-on-polyethylene articulation is the standard bearing configuration.

Cruciate-Retaining Design

In a cruciate-retaining knee replacement, the PCL is preserved.

The implant relies in part on the native PCL for femoral rollback and stability.

Posterior-Stabilized Design

In a posterior-stabilized design, the PCL is resected.

A cam-and-post mechanism built into the femoral component and polyethylene liner substitutes for part of the PCL function.

Long-term outcomes of well-performed cruciate-retaining and posterior-stabilized designs are generally similar.

Computer-Assisted and Robotic Surgery

Computer navigation and robotic-assisted systems can help improve the precision of:

Bone cuts, alignment, component positioning, and soft-tissue balancing.

These technologies are now used clinically in many centers rather than only in research settings.

Their effect on long-term implant survivorship continues to be evaluated.

Venous Thromboembolism Prophylaxis

Patients require prophylaxis against DVT and pulmonary embolism.

Modern strategies may include:

Aspirin, low-molecular-weight heparin, direct oral anticoagulants, or other agents, selected according to patient risk.

Mechanical compression and early mobilization are also important.

Routine use of warfarin for every patient is no longer standard.

Follow-Up

Referral After Surgery

Urgent reassessment is required for:

Severe or increasing pain, marked swelling, erythema, wound drainage, fever, new instability, calf pain, dyspnea, or neurovascular symptoms.

Prognosis

Long-term outcomes of total knee replacement are excellent.

Most patients experience substantial improvement in:

Pain, walking ability, function, and quality of life.

Implant Survivorship

Historical studies have reported approximately:

91% survivorship at 10 years and 78% at 20 years for older implant designs.

Newer prosthetic systems have demonstrated even higher short- and intermediate-term survival in many series.

Unicompartmental Replacement Prognosis

Older studies have reported approximately 95% survival at 10 years in selected patients.

However, arthritis may continue to progress in the unreplaced compartments over time.

Complications

Infection

Periprosthetic joint infection occurs in a small percentage of patients, historically around 1% after primary replacement.

It may require:

Antibiotics, irrigation and debridement, component exchange, or staged revision surgery.

Aseptic Loosening

Components may loosen over time because of:

Mechanical stress, wear debris, osteolysis, or failure of bone-cement or bone-implant fixation.

Polyethylene Wear and Osteolysis

Wear particles can stimulate an inflammatory reaction that leads to:

Periprosthetic bone loss and eventual component loosening.

Modern polyethylene has reduced this problem compared with older materials.

Patellofemoral Complications

Possible complications include:

Patellar maltracking, subluxation, dislocation, fracture, instability, and anterior knee pain.

Periprosthetic Fracture

Fractures may occur around the:

Distal femur, proximal tibia, or patella.

These injuries are uncommon but can be complex, particularly in osteoporotic patients.

Extensor Mechanism Rupture

Disruption may involve the:

Quadriceps tendon or patellar tendon.

This can cause major functional impairment and often requires operative repair or reconstruction.

Venous Thromboembolism

DVT and pulmonary embolism remain important postoperative risks.

Prophylaxis and early mobilization reduce their incidence.

Medical Complications

Potential medical complications include:

Myocardial infarction, pneumonia, urinary infection, delirium, and, rarely, death.

Stiffness

Some patients develop persistent loss of motion.

Contributing factors include:

Preoperative stiffness, pain, scar formation, infection, malposition, and inadequate rehabilitation.

Selected patients may require manipulation under anesthesia or further surgery.

Sensory Changes

Numbness near the incision is common because small superficial sensory nerves may be divided during surgical exposure.

The numb area may decrease with time but can persist.

Persistent Pain

A minority of patients continue to experience pain despite technically successful replacement.

Potential causes include:

Infection, loosening, instability, patellofemoral problems, referred pain, bursitis, neuroma, stiffness, or unexplained persistent pain.

Pes Anserine Bursitis

Pes anserine irritation can occur after arthroplasty and may contribute to medial knee pain.

Patient Monitoring

Patients should be followed closely during the early postoperative period until:

The wound has healed, pain is controlled, gait is safe, and functional range of motion has been achieved.

Long-Term Follow-Up

Periodic long-term review is recommended.

Plain radiographs are used to monitor:

Component position, fixation, polyethylene wear, osteolysis, periprosthetic fracture, and progressive loosening.

The frequency of surveillance varies according to implant age, symptoms, and surgeon preference.