Osteoarthritis: a degenerative disease of synovial joints that causes a progressive loss of articular cartilage
Primarily affects older individuals and has a significant economic and social burden
Hip osteoarthritis: approximately 100 per 100,000 individuals per year1
Knee osteoarthritis: 200 per 100,000 individuals per year1
Both hip and knee osteoarthritis are expected to increase in conjunction with the increase in the elderly population and the incidence of obesity.2
Difficult to place value on the social burden
Many patients report decreased enjoyment of activities or altogether missing key life events because of the discomfort imparted by arthritis of the hip and knee.
It has been estimated that osteoarthritis causes an economic burden of approximately 3% of the gross domestic product as a result of the work hours missed by patients with hip and knee pain caused by osteoarthritis.3
Both the knee and hip are classic examples of synovial joints, which allow movement and distribute the weight of the body.
Joint ends are capped with articular cartilage.
Five types of cartilage throughout the human body:
Fibroelastic cartilage, as seen in the meniscus of the knee
Fibrocartilage, often seen at tendon or ligament insertions
Elastic cartilage, found in the auricle of the external ear among other places
Physeal cartilage, found in the growth plates of bones
Hyaline or articular cartilage—focus of this chapter
Each cartilage type serves a different structural function and has slightly different composition to help facilitate the individual function.
Hyaline (articular) cartilage is composed of chondrocytes (cells) and extracellular matrix.
Embryologically, chondrocytes are derived from chondroblasts, which come from the mesenchymal stem cell lineage.
This is a complex, multistep process that involves several transcription factors that dictate stem cell differentiation, migration, and ultimate formation of hyaline cartilage.
Key transcription factor involved in this process is SOX-9.
When fully developed, the chondrocytes are responsible for the production of collagen, proteoglycans, and various enzymes.
They are also the metabolically active component of hyaline cartilage.
Chondrocytes respond to both mechanical stimuli (changes in mechanical load or hydrostatic pressure) and chemical stimuli (cytokines and growth factor response).
The noncellular aspect of hyaline cartilage is the extracellular matrix.
When listed in order of decreasing percentage of cartilage weight, this matrix is composed of water, collagen, proteoglycans, and small amounts of noncollagenous proteins.
Water makes up anywhere from 65% to 80% of the mass of cartilage depending on the layer of cartilage being sampled.
The total water content of cartilage decreases with normal aging.
However, with the pathologic process of osteoarthritis there is an increase in water content of cartilage.
This increase leads to increased permeability, decreased strength, and decreased Young modulus of elasticity.
Collagen accounts for approximately 10% to 20% of total cartilage mass.
Hyaline cartilage is composed of 90% to 95% type II collagen.
The small remaining portion is varying amounts of type V, VI, IX, and XI collagen.
The function of collagen is to provide the tensile strength and overall structural framework of the extracellular matrix.
Proteoglycans are the last major contributor to extracellular matrix composition.
They make up 10% to 15% of cartilage weight and provide the hydrophilic behavior and compressive strength of cartilage.
As their name suggests, proteoglycans are composed of a core protein heavily glycosylated with chondroitin and keratan sulfate glycosaminoglycan subunits.
Aggrecan is the most abundant proteoglycan in hyaline cartilage and is responsible for the hydrophilic behavior of the extracellular matrix.
Normal articular cartilage is composed of three distinct layers or zones—with distinct chemical composition, collagen orientation, and chondrocyte morphology—before the tidemark (Figure 1).
Outermost zone (layer in contact with synovial fluid): the superficial (tangential) zone
Characterized by flat chondrocytes and type II collagen fibers that run parallel to the surface of the joint
Has highest concentration of collagen and lowest concentration of proteoglycans of all the layers
Next layer: intermediate zone
Thickest zone of the hyaline cartilage
The collagen fibers are arranged in an oblique and often random orientation.
Final zone: deep (basal) layer
Has round chondrocytes that are arranged in columns
Has the highest concentration of proteoglycans and lowest concentration of collagen
The collagen fibers of the deep layers are arranged perpendicular to the joint surface.
Articular cartilage composition changes with normal aging, and these changes differ from the changes seen in osteoarthritis.
Although an increasingly common pathology in older adults, osteoarthritis is a distinct pathology different from the normal aging process.
Both normal aging and osteoarthritis show a decrease in the number of chondrocytes present in the articular cartilage.
The chondrocytes that remain tend to be larger and in the setting of late osteoarthritis cluster together.
The water content of the articular cartilage differs between the two processes.
Normal aging: cartilage tends to dry out, with an overall decreased water content, which, along with an increase in glycation of proteins, leads to a higher Young modulus (stiffer, less elastic)
Osteoarthritis: cartilage changes lead to a softening of the cartilage, which leads to an increased water content, resulting in a decreased Young modulus
Differences are also seen in the collagen present in the cartilage.
Normal aging: collagen becomes increasingly cross-linked, which leads to increased brittleness
Osteoarthritis: increased collagenase activity, resulting in disorganized collagen fibrils and subsequently, less brittle cartilage
Important chemical distinctions can also be made in the composition of proteoglycans and their respective glycosaminoglycan subunits.
In both normal aging and osteoarthritis, there tends to be a decrease in proteoglycan size; the difference lies in the ratio of the glycosaminoglycan subunits.
Normal aging: increased keratan sulfate to chondroitin sulfate ratio
Osteoarthritis: increased ratio of chondroitin sulfate to keratan sulfate
A pathologic process, osteoarthritis produces inflammation and degradation not seen with normal aging.
Increase in inflammatory cytokines (interleukins 1 and 6, tumor necrosis factor alpha) results in synovial inflammation, leading to a vicious cycle of joint destruction.
The inflammatory cytokines result in an increase in matrix metalloproteases such as stromelysin, which function to break down the extracellular matrix.
This increased damage results in further release of inflammatory cytokines and the cycle continues to propagate.
This cycle of joint destruction can be seen on a gross level as well.
Early stages of osteoarthritis: mild inflammatory changes to the synovium
Middle phase: synovium becomes hypervascular because of its prolonged inflamed state
As the friable cartilage becomes damaged, it leads to changes in the bone, with the subchondral bone attempting to remodel and forming sclerotic edges.
Eventually, osteophytes form from the pathologic activation of endochondral bone formation using the Indian hedgehog signaling pathway, and ultimately bone cysts develop.
Risk factors for developing osteoarthritis?
Complex and not entirely well understood concept
Few known modifiable risks factors for its development
Pathogenesis is multifactorial and linked to both systemic and local factors
Risk increases as patients age and with patient body habitus
Possible differences in patient’s risk based on sex, ethnicity, race (hormonal factors), nutrition, and even specific genes2
Systemic factors are often nonmodifiable, but it is important to have a good understanding of these factors to properly counsel patients.
Local factors, specific to a given joint
Anatomic abnormalities and malalignment
Occupation, history of injury, or even sports participation can alter the risk of the development of arthritis.
Local factors are often more modifiable, but it can be difficult to tease out the influence of local effects from the more systemic risks.
Symptoms: pain in the joint, stiffness, functional limitations, and often mechanical symptoms such as instability, locking, and catching.
Inspection: Look at body habitus, limb alignment, and check for the presence of an effusion.
Evaluate patient’s gait; patients with arthritis often have a shortened stride length and with knee arthritis an increased adductor moment (antalgic limp).
Check range of motion; range of motion often decreases as arthritis progresses.
Evaluate the patient holistically, keeping in mind comorbidities because they can ultimately affect treatment options and outcomes.
Radiography: main imaging modality
Hip: AP view of the pelvis and lateral view of the affected hip (Figure 2)
Some physicians will recommend a false profile view or cross-table lateral view.
The lumbar spine also should be evaluated because this can affect symptoms and treatment.
Knee: AP, lateral, and patellofemoral view weight-bearing views (Figure 3)
Radiographic findings of osteoarthritis: joint-space narrowing, osteophyte formation, subchondral sclerosis, and cysts
Advanced imaging such as MRI is rarely beneficial and should not be routinely ordered in the setting of osteoarthritis.
Osteoarthritis is defined as a degenerative disease of synovial joints that causes a progressive loss of articular cartilage.
Hip and knee osteoarthritis is a major cause of disability in the United States.
Articular cartilage is found in synovial joints and is one of the five main types of cartilage.
Chondrocytes are derived from chondroblasts and are the main cell in articular cartilage.
SOX-9 is the main transcription factor involved in chondrocyte differentiation.
Osteoarthritis results in increased water content of the cartilage.
Type II collagen is the main type of collagen found in articular cartilage.
Aggrecan is the most abundant proteoglycan in articular cartilage.
The main patient complaint of osteoarthritis is pain.
The main imaging modality for diagnosis of arthritis is radiographs; advanced imaging is rarely needed.
Key findings in patient history: pain that is better with rest and worse with increased activities, stiffness, swelling, subjective instability or giving away of the knee, and history of prior knee injury or surgery
Key findings on physical examination: progressive varus or valgus deformities, an antalgic gait, limp, muscle atrophy, effusion, joint line or patellofemoral tenderness to palpation, painful and limited range of motion, and crepitus
Certain activities classically affected in patients with DJD of the knee: pain with walking and weight bearing; difficulty descending steps, standing from the seated position, and getting in and out of a car
Radiographs: decreased joint space, osteophytes, subchondral sclerosis, and bone cysts
Health and behavior modifications: physical therapy, weight loss, and knee braces
Pharmacotherapy: acetaminophen and topical and oral NSAIDs are strongly recommended, whereas glucosamine and/or chondroitin sulfate have a limited recommendation
Intra-articular injections: corticosteroids are recommended. Hyaluronic acid injections are not recommended.
Knee replacement surgery is typically reserved for those patients in whom several methods of nonsurgical treatment have failed.
Surgical management for DJD of the knee can be further subdivided into nonarthroplasty and arthroplasty options.
Nonarthroplasty options: osteotomy, arthroscopy, and synovectomy
Arthroplasty options: unicompartmental knee arthroplasty, TKA
Unicompartmental knee arthroplasty
Indications as delineated by Kozin et al4 and Kozin and Scott:5 isolated medial compartmental DJD (no lateral compartment arthritis, only mild patellofemoral arthritis on Merchant view); no lateral joint line tenderness; intact anterior cruciate ligament (in wear pattern on lateral radiograph of the knee, posterior tibial bone loss indicates disrupted anterior cruciate ligament because the tibia has shifted anteriorly without the anterior cruciate ligament restraint, exposing the posterior tibia to contact stresses from the distal femoral condyles); noninflammatory arthritis; weight
less than 82 kg; correctable varus deformity (less than 5° deformity); patient age older than 60 years; flexion contracture less than 5°; range of motion greater than 90°
These criteria served as the foundation for patient selection for years, yet recent studies and surgeons have questioned those previously established thresholds and have expanded indications to include no age limit, higher body mass index, and acceptance of greater angular deformity measurements (increasing the flexion contracture to less than 15°, and less than 10° varus or 5° valgus deformity)6
Technique of TKA
Classically, anterior longitudinal midline incision made from approximately three to four fingerbreadths above the superior pole of the patella proximally to the inferior medial aspect of the tibial tuberosity distally
General principles to consider for TKA incisions: prior scars should be taken into consideration to preserve vascularity, preexisting anterior longitudinal scars should be incorporated when possible, and when parallel anterior longitudinal scars are present, the one most lateral should be used if possible
If it is not possible to use a prior incision, a wide skin bridge of at least 8 cm should be allowed between the new incision and previous scar.
Horizontal scars can be crossed at right angles, and short oblique scars may be ignored.
After the skin incision has been made, full-thickness skin flaps are created and then an arthrotomy can be performed.
Most common arthrotomy approaches: medial parapatellar, subvastus (Southern), and midvastus
Medial parapatellar approach: quadriceps tendon is cut longitudinally from proximal to distal along its medial border, leaving a cuff of tendon 5 to 10 mm wide, then the arthrotomy is carried further distally, skirting along the medial border of the patella and patellar tendon
Subvastus approach: blunt dissection is carried from the medial intermuscular septum; a transverse incision is made at the midpatella through the medial retinaculum inferior to the vastus medialis and is stopped once the patellar tendon is reached, and then a second incision is made along the medial border of the patellar tendon to the tibial tubercle
Midvastus approach (Figure 4): blunt finger dissection is begun at the superomedial pole of the patella in the midsubstance and through the full thickness of the vastus medialis muscle, and is extended parallel to its fibers, to a maximum of 4 cm proximal medial to this starting
point, then the incision is taken similar to the previous approaches distally along the medial border of the patellar tendon to the tibial tubercle
The decision regarding which approach to use is determined by surgeon preference, as the most current data indicate that the results following TKA using any of these approaches are similar.7
Either the femur or the tibia can be prepared first for a TKA
Standard bone cuts for any TKA:
Distal femoral condylar resection
Anterior and posterior condylar resections
Anterior and posterior chamfer resections from the distal femur
Transverse proximal tibial resection
Retropatellar cut in patellar resurfacing
Intercondylar box cut—performed only for posterior-stabilized designs
The order of the bone cuts and decision of whether to use a cruciate-retaining TKA or posterior-stabilized (cruciate-substituting) TKA is determined by surgeon preference.
Classically, bone cuts are made to align the implanted knee prosthesis perpendicular to the mechanical axis of the lower extremity, thereby distributing weight-bearing forces evenly between medial and lateral compartments (Figure 5).
Some surgeons have begun advocating for making bone cuts relative to the kinematic axis of the knee, a technique that seeks to restore the native tibia varus and femoral valgus as opposed to referencing the mechanical axis of the lower extremity.8
Femoral component rotation
Making accurate cuts is essential to obtaining proper size and rotation of the final femoral component, and achieving appropriate femoral rotation is critical for patellar tracking.
There are four basic techniques to setting femoral rotation.
Three use femoral anatomic landmarks and are known as measured resection techniques for achieving appropriate femoral rotation.
Perpendicular to the Whiteside line (the trans-sulcus line, which is a line drawn from the top of the intercondylar notch to the deepest part of the femoral trochlea)9
Parallel to the transepicondylar axis
Aligning the cutting block in 3° of external rotation relative to the posterior condylar axis (Figure 6)
The fourth technique—gap balancing—is independent of femoral anatomic landmarks, instead using the flat proximal tibial resection and ligament balance to set femoral rotation.
Most surgeons use a combination of all four techniques and multiple reference points to help reduce any error in setting the rotation of the femoral component.
At least three ways have been described to orient the rotational alignment of the tibial component.
The first method is to use an asymmetric or anatomic tibial tray that mimics the cut surface of the tibia and apply the tray anatomically.
The second method is to align the tibial rotation based on the tibial tubercle; the most commonly used landmark is the junction between the medial and central thirds of the tubercle.
The third method is known as floating the tibia component during the trial range of motion, which allows the fixed femoral component articulation to set the rotation of the tibial component to achieve proper patellar tracking.
No matter which method is chosen, it is important to avoid internal rotation of the tibial component, because this can result in significant patellar tracking problems.
Often, but not always performed during TKA
Patellar preparation can be done at any point in the procedure.
Preparation immediately following the initial approach can facilitate exposure for the rest of the procedure.
The patella cut can be performed using a patellar cutting jig, mill, or freehand technique with an oscillating saw.
Regardless of technique, the goal is to make a flat cut and place the patellar button superiorly and medially to help achieve optimum patellar tracking within the femoral trochlea.
A caliper is used to assess the patellar thickness before the cut and after the patella is resurfaced.
Care must be taken to avoid overstuffing the patellofemoral compartment, as this can adversely affect flexion and tracking.
A minimum of 12 mm of patellar bone stock must remain after patellar resection, because less than 12 mm of bone stock is associated with a higher risk of postoperative patella fracture and osteonecrosis.
Balancing the knee
After component positioning is achieved, trial component insertion and reduction are performed.
At this time, final balancing of the knee is assessed and any necessary adjustments are performed to ensure flexion and extension gap symmetry.
The flexion gap is the space created by the tibial cut surface and the posterior femur.
The extension space is created by the tibia and distal femur.
The collateral ligaments form the sides of these rectangular spaces.
The goal of balancing the knee is to achieve equal flexion and extension gaps with symmetric tension on the collateral ligaments.
Table 1 provides a general guide to aid in correcting gap asymmetries during the trial phase.
TKA components can be fixed with either bone cement or press-fit technique.
Cementation is currently the primary mode of TKA fixation in the United States, but press-fit designs are gaining popularity.
When cement technique is used, polymethyl methacrylate is used for fixation of the components.
Before cementation, it is important to prepare the cut bony surfaces by using pulsatile lavage to thoroughly irrigate the bone, remove all debris, and then subsequently dry the bone completely with suction and dry gauze.
It is important to remove all excess cement particles from the joint to avoid third-body wear.
Although cemented fixation of TKA components remains the gold standard, increasing demand among young and active patients, coupled with an increasing life expectancy, has spurred growing interest in the use of press-fit options.10
TABLE 1 Techniques to Balance the Flexion and Extension Gaps in Total Knee Arthroplasty
Use thinner tibial insert
Resect additional tibia
Augment distal femur
Anteriorize femoral component
Downsize femoral component (anterior referencing system only)
Release posterior soft tissues
Resect additional distal femur
Augment posterior only
Use thicker tibial insert
Midterm to long-term studies have demonstrated no significant differences in the clinical outcomes between the cemented and cementless groups.
One such study concluded that over a minimum 8-year postoperative period the mean ranges of knee movement and radiologic results were similar in both groups, no osteolysis was identified in either group, and the rate of survival of the femoral and tibial components was 100% in both groups at final follow-up.11
Constraint in implant design
One of the keys to long-term success in TKA is knee joint stability.
Soft-tissue balancing in the coronal plane is performed by releasing the contracted ligament on the concave side of the deformity.
The extent of the releases will be based on the degree of deformity.
Cruciate-retaining or cruciate-substituting (posterior-stabilized) implants (Figure 7)
Figure 7 A, AP and lateral (B) radiographs demonstrating a cemented posterior-stabilized total knee arthroplasty.
After the coronal plane stability has been established, standard implants, including cruciate-retaining or posterior-stabilized knee implants, may be used.
A posterior-stabilized implant provides stability in the sagittal plane and prevents posterior translation of the tibia relative to the femur, but it does not provide an increase in coronal plane stability.
It can be difficult to obtain adequate coronal plane soft-tissue balancing in severely deformed knees.
When ligament balancing is not possible and there is persistent varus or valgus laxity, more constraint is required.
In this setting, a constrained prosthesis design should be used to prevent medial or lateral instability and recurrent deformity.
Constrained designs have tibial posts that are higher and wider in comparison with standard posterior-stabilized designs.
Useful for coronal plane instability where a collateral ligament is still present although lax
In the setting of a severely damaged knee where there is an incompetent or absent ligament, or hyperextension, a constrained design will not provide adequate stabilization of the joint, and a hinge design must be used.
The hinge design allows for the restoration of knee joint stability that the other less constrained designs cannot provide in the severely damaged knee, particularly when there has been massive bone loss and absent ligament support.
Gradations in constraint improve stability and are useful in certain settings, but it is also important to keep in mind that increasing constraint leads to increased loads to the implant fixation interfaces, which can carry a risk for earlier failure rates.
The least amount of constraint necessary should therefore be used. Still, joint stability is one of the key goals to long-term success of TKA.12
Rehabilitation protocols vary but typically include early mobilization and initiation of weight-bearing activities, early range of motion, appropriate pain management, and weaning off assistive devices as tolerated.
Current analgesic strategies use a multimodal approach to pain management.
Antibiotics are typically administered for one or two doses postoperatively.
Patients should also be placed on a venous thromboembolism prophylaxis protocol, although the exact regimen used varies by surgeon.
Patients are typically discharged, either to home or to an inpatient rehabilitation facility.
Surgeries can also be performed on an outpatient basis in appropriately selected patients.
Once strength, mobility, and balance are regained, patients can resume low-impact sport activities such as cycling, swimming, walking, hiking, golf, or bowling.
Higher impact activities such as basketball, soccer, and football are generally discouraged.
Survivorship of TKA is related to appropriate alignment and balance.
In patients with correct positioning of the tibial and femoral components both axially and rotationally, there was considerable improvement in functional outcome and excellent long-term clinical survivorship.13
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