Medicine

Soft Tissue Knee Injury

Soft Tissue Knee Injury

Practice Essentials

Soft tissue knee injuries are some of the most common and clinically challenging musculoskeletal disorders seen in the emergency department. Accurate and timely diagnosis increases the likelihood of fully restoring normal and pain-free use of the affected knee.

Essential update: Physical therapy as effective as surgical therapy for meniscal tear

Results of the Meniscal Tear in Osteoarthritis Research (METEOR) trial indicated that physical therapy may be as effective as surgery for the treatment of meniscal tear. In the study, which enrolled 351 patients with documented osteoarthritic cartilage change and at least 1 symptom of meniscal tear, 174 patients were randomized to arthroscopic partial meniscectomy plus postoperative physical therapy, and 177 were randomized to physical therapy alone. At 6 months, there were no significant differences between groups with respect to Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) function score, pain improvement, or frequency of adverse events. Patients in the physical therapy−only group were allowed to cross over to the surgery group at any time; 30% of them did so. Results at 12 months were similar to those seen at 6 months.

Signs and symptoms

The history should focus on the following:

· Confirming that an acute traumatic event occurred

· Documenting the mechanism of injury (including the circumstances of the injury)

· Assessing the stability of the joint

· Determining the type and location of the pain

· Evaluating the onset and degree of knee effusion

· Assessing joint mobility (eg true locking vs pseudolocking)

The initial physical examination focuses on inspection, palpation, and neurovascular evaluation. Steps that may be useful include the following:

· Observation of the patient’s stance and gait, if possible

· Inspection of the knee

· Ballottement of the patella

· Measurement of the Q angle

· Inspection and palpation of the popliteal fossa

· First and second Steinmann signs

· Payr sign

· Patellar apprehension sign

· Palpation of the anterior aspects of both thighs (in quadriceps rupture)

· Palpation of the relevant bursae (in bursal injuries)

· In children, palpation of the epiphyses

· Range-of-motion (ROM) testing

· Lachman maneuver

· Anterior and posterior drawer tests

· Tibial sag test

· Pivot-shift test

· McMurray test

· Apley compression test

· Thessaly and Ege tests

Diagnosis

The following laboratory studies may be helpful:

· Blood typing and screening

· Complete blood count (CBC)

· Erythrocyte sedimentation rate (ESR)

· Serum electrolytes

· Blood glucose

· Blood urea nitrogen (BUN) and creatinine

· Analysis of aspirated synovial fluid for white blood cell (WBC) count and differential and for glucose and protein levels

Plain radiographs are recommended for the following scenarios:

· Patients older than 55 years

· Patients with tenderness over the fibular head

· Patients with discomfort confined to the patella upon palpation

· Patients unable to flex the knee to 90°

· Patients incapable of bearing weight, immediately and in the ED, for at least 4 steps

Other diagnostic procedures that may be helpful are as follows:

· MRI

· Ultrasonography

· Arteriography (when a knee dislocation is strongly suspected)

· Injection of methylene blue

· Knee joint aspiration

Management

General treatment principles include the following:

· Aside from the particular injury, treatment depends on the patient’s age and activity level and the presence of additional injuries

· Obtain orthopedic consultation when appropriate

· Initial nonpharmacologic treatment includes rest, ice, compression, and elevation (RICE)

· For the first 1-3 days, use therapeutic measures that minimize incipient damage and reduce pain and inflammation

· Consider splinting the injured knee to provide support and to prevent further injury

· Serviceable devices include immobilizers and handcrafted compressive dressings

· Detrimental effects of immobilization include joint stiffness, degenerative changes in articular cartilage, muscle atrophy and weakness, and decreased vascularity

Therapy for specific injuries include the following:

· For first-degree sprains, provide symptomatic treatment (essentially RICE); normal function usually returns quickly

· Second-degree sprains must be protected by using a cast, cast brace, or a restrictive movement brace; arrange for timely follow-up care

· Treatment of third-degree sprains depends on the severity and type of instability; some third-degree sprains of ligaments call for surgical repair

· Treatment for anterior cruciate ligament (ACL) injuries is individualized, influenced by the presence or absence of comorbid pathology, age of the patient, baseline activity level, degree of instability, and associated ligamentous injuries miring the knee

· Presence of a meniscal tear does not automatically lead to surgical intervention; if the knee is not locked or unstable, conventional treatment (ie, RICE) ordinarily suffices

Reduction of knee dislocation includes the following:

· Most knee dislocations reduce spontaneously before arrival in the ED; however, when findings suggest neurovascular compromise, attempt to restore circulation with traction or reduction and emergency orthopedic consultation

· Classify dislocations with respect to the relationship of the tibia on the femur

· Anterior dislocations occur most commonly

· Ideally, perform reductions in the operating room with general anesthesia; however, if circumstances preclude this scenario, an attempt in the ED is warranted

· Barring contraindications, administer conscious sedation

· With an assistant providing stabilization and countertraction of the thigh, a second person applies longitudinal traction to the leg; this usually suffices for reduction.

· Reduction of an anterior dislocation may be aided by transposing the femur anteriorly

· Avoid affixing pressure over the popliteal space; this may exacerbate arterial damage

· For posterior dislocations, attempt to reinstate the tibia anteriorly

· After relocation, confirm neurovascular status and immobilize the knee in 15° of flexion

· Order postreduction images and consultation with the orthopedic surgeon, and obtain an emergency arteriogram

Reduction of patellar dislocation includes the following:

· Patellar dislocations typically occur in predisposed individuals and tend to recur

· Patellar dislocations are identified with respect to the patella’s position on the knee joint, with lateral dislocations being most common

· If the joint has not reduced spontaneously, verify the dislocation radiographically

· After administering necessary analgesia, place the hip in a mild amount of flexion, and gently press anteriorly and medially on the patella while extending the knee joint

· Postreduction films should include a sunrise view

· Other types of patellar dislocations tend to be resistant to closed reduction

· Aspiration of a Baker cyst may render temporizing relief

Image library

Pivot test.

Background

Soft tissue injuries of the knee are some of the most common and clinically challenging musculoskeletal disorders in patients presenting to the ED. Annually, more than 1 million emergency department (ED) visits and 1.9 million primary care outpatient visits are for acute knee pain. Therefore, establishing clear-cut diagnostic and therapeutic objectives for these injuries is important.

Knee pain and related symptoms may derive from damage to one or more of the soft tissue structures that stabilize and cushion the knee joint (including the ligaments, muscles, tendons, and menisci), from infection to the knee joint or surrounding structures, or from trauma to the bones forming the joint. In addition to the etiology of the patient's presenting symptom, determine the acuity of the pathologic process as an acute traumatic or infectious event or exacerbation of a chronic overuse or degenerative syndrome.

Accurate and timely diagnosis increases the likelihood of fully restoring normal and pain-free use of the affected knee. For most patients, the severity of the etiology and the injury or pathologic process, acute or chronic, can be determined from a targeted history, focused physical examination, and thoughtful workup including diagnostic imaging (eg, plain radiography).

Pathophysiology

To understand the various injury patterns associated with trauma to the knee, understanding the anatomy is important. The anatomy of the knee is shown in the image below.

Anatomy of the knee.

Knee symptoms arise from an alteration or disruption of the normal anatomic structures that impede normal knee function. In mechanical terms, the knee performs like a rolling cam rather than as a simple hinged (ginglymus) joint. As the knee proceeds from flexion to extension, a complex screw-type of motion takes place, with the femoral condyles locking into the tibial plateau as the femur rotates internally. Full knee extension increases the tautness of the major bracing ligaments, transforming the knee into a mechanically rigid structure. Flexion loosens the knee joint by unlocking and disengaging the bracing structures, including retraction of the menisci, thereby enhancing ligamentous laxity and increasing the range of motion (ROM) of the joint.

Two separate but interdependent joints forming the knee are the tibiofemoral articulation and the patellofemoral coupling. Weight-bearing forces, as much as 5 times an individual's body weight, are transmitted through the opposing condyles of the femur and the tibia. Two shock-absorbing cartilaginous menisci interpose between the femur and the tibia, forming the largest synovial joint in the body. The medial meniscus is smaller and more fixed than the lateral meniscus; these features predispose it to injury. A fibrous capsule lined by a synovial membrane also surrounds and bolsters the knee joint but does not contribute to the inherent stability of the joint.

Fitness of the knee joint largely depends on the fortifying ligaments and muscles binding together the femur, tibia, and patella. Two sets of knee ligaments are frequently affected. The first set, lying outside of the knee joint proper, are the extracapsular collateral ligaments. These ligaments consist of the medial collateral ligament (MCL), which opposes extreme abductive and/or valgus forces, and its counterpart, the lateral collateral ligament (LCL), which limits excessive adductive and/or varus pressures. The second set, crisscrossing in the knee joint, are the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL), which individually brace against excessive translation in the anteroposterior (AP) plane. The ACL serves as the primary knee stabilizer, preventing forward displacement of the tibia on the femur.

Primarily formed by the quadriceps muscles, the extensor apparatus envelops and stabilizes the patella. At its distal aspect, the quadriceps muscle consolidates into the patellar ligament, ultimately inserting onto the tibial tubercle. Several bursae envelop the knee, including the prepatellar, superficial and deep infrapatellar, and pes anserine bursae, which permit friction-free movement between the various structures. Acute trauma or repetitive occupational stress may incite inflammation; infection and metabolic disorders (eg, gout) are less common etiologies. Inflammation of the bursa then leads to localized tenderness, erythema, and increased warmth. Extensive bursae in this area alleviate potentially damaging frictional forces between the susceptible structures. Fixed in the back of the knee joint, in the popliteal fossa, are vital neurovascular structures, including the popliteal artery.

Definitions and grades of sprains and strains

Sprains to the knee are characterized by the stretching or tearing of noncontractile structures, such as the investing ligaments or of the joint capsule itself, whereas a strain refers to stretching or severing along the course of muscles or tendons. Both collateral ligament and cruciate ligament sprains, as well as muscular strains, are relatively common. Ligamentous (sprain) and muscular (strain) injuries may be classified according to the degree of impairment.

· Grade I sprain - Stretching but no tearing of the ligament, local tenderness, minimal edema, no gross instability with stress testing, firm end point

· Grade II sprain - Partial tears of the ligaments, moderate local tenderness, mild instability with stress testing (but firm end point), moderately incapacitating

· Grade III sprain - Complete tear, discomfort with manipulation but less than expected for degree of injury, variable amount of edema (ranging from negligible to grossly conspicuous), clear instability with stress testing (expressing a mushy end point), severe disability

Specific injuries

· ACL injury: Rupture of the ACL is among the most serious of the common knee injuries and results from a variety of mechanisms. Most patients with ACL damage complain of immediate and profound pain, exacerbated with motion, and inability to ambulate. Disruption of the ACL may occur alone or with other knee injuries, especially a meniscal injury or tear of the MCL.

· PCL injury: Patients typically report falling on a flexed knee or sustaining a direct blow to the anterior aspect of the knee (eg, when the knee strikes the dashboard in a motor vehicle accident). PCL harm signifies a major injury and rarely occurs as an isolated injury.

Frequency

International

As in the United States, the incidence of knee injuries is increased in countries where sporting activities, such as skiing, soccer, and basketball, are popular.

Mortality/Morbidity

Patients receiving inappropriate or ill-timed care of knee dislocations may have undue morbidity (eg, amputation) due to vascular complications in the distal leg. Oversight of the magnitude of soft tissue injuries of the knee may result in a failure to expeditiously consider compartment syndrome and its resultant complications, including loss of a limb. Misdiagnosis or mismanagement of damage to supporting structures of the knee may lead to chronic knee instability with subsequent development of degenerative joint disease and/or loss of knee function, including but not limited to an inability to bear weight or ambulate.

Sex

Disorders of the patella and lateral meniscus are generally more common in girls and women than in boys and men. Recent studies also suggest that females are more prone to ACL injuries, which is believed to be due to the fact that the female ACL is both structurally weaker and has a relatively smaller cross sectional diameter. Chondromalacia patellae or patellar malalignment syndrome (ie, premature erosion and degeneration of patellar cartilage) predominates in young women.

Larsen-Johansson disease of the patella, also known as inferior pole patellar chondropathy, is 9 times more prevalent in boys and men than in girls and women, especially in boys aged 10-14 years.

Age

Ligamentous and meniscal injuries are most likely in young to middle-aged adults, whereas children and adolescents are most susceptible to osseous damage. Most patients with a meniscal tear are aged 20-30 years, but a second peak occurs in patients older than 60 years. Meniscal injuries are rare in children younger than 10 years with morphologically normal menisci.

In general, knee dislocations arise from high-energy trauma, such as motor vehicle accidents. The epidemiology of injury follows that of car accidents; therefore, patients tend to be young men.

Overall, 18.1% of US men and 23.5% of US women aged 60 years and older reported knee pain on most days for 6 weeks prior to their medical examination. Additionally, elderly patients may sustain fractures after minimal trauma that typically produces only soft tissue injuries in younger patients.

The region of the extensor mechanism susceptible to disruption is correlated with the patient's age. The older the patient, the more proximal the area of rupture. Disruption of the quadriceps tendon most often occurs in elderly patients, whereas more distal severance of the patellar tendon and avulsion of the tibial tubercle occurs in younger patients.

Anterior Cruciate Ligament Pathology

Background

The anterior cruciate ligament (ACL) is one of the most commonly injured ligaments of the knee. Injuries occur predominantly in a young and sports-active population. Many patients are left with significant disability following injury to the ACL. Understanding and preventing associated meniscal pathology is the key to management of this condition. This article endeavors to explain the complex nature of the ligament and its injuries and aid the reader in making informed management decisions.

History of the Procedure

Bonnet first discussed ACL injury in the medical literature in 1845. Further discussion was made by Segond in 1879. Stark made the first reports in the English literature in 1850. In 1900, Battle described surgical intervention when he attempted repair of the ACL. Subsequent operative descriptions include those by Groves and Jones in 1913.

In 1917, Groves described a reconstructive procedure. He used the iliotibial band (ITB) as the graft. In 1918, Smith detailed combined intra-articular and extra-articular procedures. Zur Verth first used the patellar tendon graft in 1933. Campbell used this same donor tissue in 1936. In 1966, Bruckner reported using the patellar tendon as a free graft. Since that time, numerous entries have been made in the literature describing natural history, operative reports, and surgical series.

Problem

Anterior cruciate ligament injury often occurs in the young and sports-active population. See the images below.

MRI is used as an aid to diagnose anterior cruciate ligament injury. MRI has a sensitivity of 95%

Anterior cruciate ligament reconstruction aims to reduce instability episodes in an attempt to preserve the meniscus. When meniscal injury has occurred, the knee becomes degenerate with time.

Mechanics of the knee are altered following injury. This mechanical deficit can lead to an increased risk of meniscal injury. Incidence of osteoarthritis rises sharply when the meniscus is injured. Treatment aims to protect the meniscus by modifying activity levels or reconstructing the ACL.

Epidemiology

Frequency

Incidence of anterior cruciate ligament injury in the United States is estimated to approach 1 case per 3000 individuals. Over 100,000 injuries occur per year from snow skiing in the United States alone. Estimated cost of management is in the order of 2 billion dollars annually, which is a significant problem. The authors' series has demonstrated an incidence of 1.5% of the population of New South Wales, Australia, with males affected twice as often as females.

Females are at higher risk of ACL injury when considering sports participation numbers. This is believed to be related to both intrinsic factors (increased Q angle, decreased notch width, increased joint laxity, hormonal influences) and extrinsic factors (less muscle strength, different muscle activation patterns, altered cutting and landing patterns). Further investigation is required to fully identify which of these factors are the most important and if any alterations to ACL injury patterns can be made as a result of intervention.

Etiology

Approximately 70% of anterior cruciate ligament injuries occur through noncontact mechanisms. Patients experience giving way of the knee when attempting to rapidly change direction. This involves deceleration, coupled with a cutting, pivoting, or sidestepping maneuver. The remainder of cases tend to occur through direct contact and often are associated with other ligament injury.

Pathophysiology

The anterior cruciate ligament acts as the primary restraint to anterior tibial translation and guides the screw-home mechanism associated with knee extension. The ACL acts secondarily to prevent varus and valgus, particularly in the extended knee. Injury leads to abnormal kinematics of the knee. Subluxation episodes occur, creating abnormal shear forces on the meniscus and articular cartilage. Subsequent meniscal injury therefore is increased significantly. The authors have found a significant increase in this meniscal pathology with delay of ACL reconstruction. Associated with this meniscal pathology is an increased incidence of osteoarthritis. A series conducted by the authors demonstrates a 15% incidence of ACL tears in patients undergoing total knee replacement (TKR). This incidence is at least 3 times the incidence of ACL tears found in the general population.

Presentation

Clinical presentation of anterior cruciate ligament injury occurs either as an acute injury or as a more chronic problem of recurrent instability.

Acute injury

Patients with acute injury present following an instability episode as previously described. More than 50% of these patients state that they felt a pop as the injury occurred. Up to 80% of these patients develop a rapid hemarthrosis (see the image below) over the subsequent 4 hours. Some studies demonstrate that occurrence of ACL injury may be as high as 70% in all hemarthroses and must be excluded in such injuries. See the image below.

Hemarthrosis of the knee occurs early following injury to the anterior cruciate ligament.

Associated injury is common. Meniscal tears occur in approximately 50% of cases, with a slightly higher incidence of lateral meniscal tears when compared with medial meniscal injury. Other pathology includes bone bruising in up to 70% of cases (mostly of the lateral femoral condyle), medial collateral ligament injury, and true fractures of the tibial plateaus or femoral condyles.

Clinical signs in the acute phase include a large hemarthrosis, limited range of motion (ROM), and joint-line tenderness.

Special tests can aid in making the diagnosis. A positive Lachman test (see the first image below), dynamic extension test (see the second and third images below), and anterior drawer test (see the fourth image below) assess anterior tibial translation. KT1000 assessment with more than 3 mm of translation compared with the other knee can quantify the degree of instability. Pivot-shift and jerk tests (see the fifth image below) assess rotational instability but are difficult to perform, particularly in early postinjury stages. The anterior draw test may be negative in up to 50% of cases, as the posterior horn of the medial meniscus can directly block translation. This feature is well described by DeHaven.

See the images below.

The Lachman test is a sensitive test for examining the cruciate-deficient knee.

Dynamic extension test produces anterior tibial translation in the cruciate-deficient knee.

This image demonstrates the anterior tibial translation compared to that seen in the image above.

The anterior draw test commonly is performed to diagnose anterior cruciate ligament injury.

The pivot-jerk test is a sensitive test for examining anterior cruciate ligament integrity.

Chronic ACL instability

Chronic presentations tend to involve repeated instability episodes, particularly when attempting the cutting maneuver.

Presentation also may be associated with mechanical pain related to meniscal tears or early osteoarthrosis.

Recurrent effusions and a history of locking also are important points in assessing associated injury.

In chronic presentations, a similar assessment is made. Meniscal pathology should be sought with joint-line palpation and the McMurray maneuver. Pivot-shift and jerk tests are performed more easily and can provide a better indication of the degree of rotatory instability. The Lachman test can be negative in chronic cases. The ACL can scar to the posterior cruciate ligament (PCL), thus limiting anterior translation of the tibia. As the scarring is nonanatomic, the pivot-shift phenomena persists, as does symptomatic instability. This should be kept in mind in chronic cases.

Indications

The aim in treating patients with anterior cruciate ligament injuries is to prevent recurrent instability and associated meniscal injury. Once meniscal pathology occurs, there is a much higher incidence of osteoarthritis.

Following acute injury, the major indications for surgical reconstruction are related to the degree of instability and level of activity. Activity levels can include sporting or work-related activities. Reviewing the patient profile in the history from the patient is very important in the decision-making process.

Daniel defines patients at highest risk as those who participated in more than 50 hours of high-level activity per year prior to injury and have marked instability.^{[1, 2]}Instability was defined as having a KT-1000, manual max test of greater than 5 mm of anterior translation. In those with translation of greater than 7 mm, this was defined as marked instability. Surgical intervention can be justified in these cases, as these patients were found to be at greatest risk of requiring further surgery.

In the less active group, the decision for surgery becomes more controversial. A nonoperative management plan with extensive physical therapy and activity avoidance can be undertaken. If patients experience ongoing instability or are unwilling to modify activity levels, surgery should be considered.

In chronic cases, the major indication for surgical reconstruction is recurrent instability. Other types of surgical intervention may be required to deal with associated pathology, such as arthroscopy, partial meniscectomy, or meniscal repair. High tibial osteotomy also may be required to correct varus alignment, especially in degenerative cases and complex instabilities.

When surgery is indicated there are many decisions to make, including the following:

· Timing of surgery

· Surgical approach

· Type of graft

· Rehabilitation (Return to activity and sport)

Early surgery is indicated in the active and professional population because it ensures future stability and negates the possibility of future giving-way episodes, protecting the menisci. Early surgery is thought by some to cause postoperative stiffness and arthrofibrosis. However, early rehabilitation negates these concerns.

Relevant Anatomy

Anatomy of the anterior cruciate ligament is very complex. The ligament is intra-articular but extrasynovial. The ACL is described as being composed of 3 main bundles. These bundles include the anteromedial, posterolateral, and intermediate. The ACL really functions as a continuum, with a portion being tight through all ranges of knee flexion.

The ligament courses obliquely, running from the tibia anteriorly and medially to the femur posteriorly, superiorly, and laterally. The broad tibial footprint lies at a point one third to one half the distance between the medial and lateral tibial spines, 5–7 mm anterior to the posterior cruciate ligament (PCL). On the femoral side, the attachment lies on the medial aspect of the lateral femoral condyle, just anterior to the posterior aspect of the intercondylar notch. An intercruciate ligament joins the ACL to the PCL. This intercruciate ligament may have some role in proprioception and coupling of the 2 ligaments.

The microstructure of the ligament is composed of collagen fiber bundles, grouped into fascicles. Type I collagen is the predominant collagen type, comprising over 90%. Types III and VI also are found. Elastin is found in significant amounts and provides some of the elastic properties of the ligament.

The major blood supply for the ACL comes from the synovium and fat pads. The middle geniculate and terminal branches of the inferior medial and lateral geniculate vessels are the vessels involved.

Sensory receptors and nerve fibers have been identified in the ligament and associated feeding blood vessels. This suggests some sensory role and possible proprioceptive function.

Contraindications

Surgical contraindications are limited and include the following:

· Active infection

· Soft-tissue abrasion

· Patient reluctance to participate in the complex rehabilitation required

However, relative contraindications are common and include the following:

· Patient is less than 2 weeks from injury

· Low activity levels

· Preexisting osteoarthrosis

· Skeletal immaturity

· Inflammatory arthropathy

Hip Reconstruction

THA Prosthesis Design

Introduction

Biomechanics of total hip arthroplasty depends on

prosthesis design
bearing surface and lubrication characteristics
fixation method

History

Austin-Moore is credited with the design of the first hemiarthroplasty

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classic Charnley design was cemented polyethylene cups with a cemented femoral stem

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acetabular cups are now almost always press-fit as prior cemented sockets had poor results

cemented cups fail due to high shear and tension forces

femoral stems can be cemented or press-fit
hybrid fixation: press-fit acetabular cup with a cemented femoral stem

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Press-fit Femoral Stem

Overview

relies on biologic fixation
compression hoop stresses provide initial stability

Unique complications

intraoperative fracture

more likely in press-fit
typically due to underreaming

loosening

high loosening rate when used in irradiate bone (due to lack of ingrowth)

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Cemented Femoral Stem

Overview

relies on cement fixation

cement is a grout that provides initial and long-term stability
limited remodeling potential
cement preferred for irradiated bone due to the bone's limited ability for ingrowth

Unique complications

stem breakage

cemented stems are smaller than press-fit stems and unable to tolerate as much cantilever bending

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Bearing Surfaces

Metal on Polyethylene

metal (cobalt-chrome) femoral head on polyethylene acetabular liner
benefits

longest track record of bearing surfaces
lowest cost
most modularity

disadvantages

higher wear and osteolysis rates compared to metal-on-metal and ceramics
smaller head (compared to metal-on-metal) leads to higher risk of impingement (smaller head:neck ratio)

Metal-on-Metal

benefits

better wear properties (lower linear wear rate and volume of particles) than metal on poly,
larger head allows for increased ROM before impingement (large head:neck ratio)

disadvantages

more expensive than metal-on-poly
increased metal ions in serum and urine (5-10x normal)

serum metal ion concentration highest at 12-24 months

correlates with the initial "wear in" or "run-in" phase of increased particle generation, but then followed by a "steady state" phase of decreased particle generation

no proven cancer link
contraindicated in pregnant women, persons with renal disease, and those with metal hypersensitivity due to metal ions

formation of pseudotumor

Ceramic on Ceramic

benefits

best wear properties of all bearing surfaces
lowest coefficient of friction of all bearing surfaces
inert particles, no concern for cancer risk

disadvantages

more expensive than metal-on-poly
worst mechanical properties (brittle)
reports of squeaking in certain types of ceramic THA
less modularity (fewer neck length options)
stripe wear - caused by contact between the femoral head and rim of the cup during partial subluxation, results in a crescent shaped line on the femoral head

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Titanium on Polyethylene

titanium has low surface hardness and is easily scratched
titanium heads not used in THA due to having the highest wear rates

Lubrication

Hip lubrication types include

boundary lubrication

the lubricant (synovial fluid) is not thick enough to prevent some contact between the surfaces, but separates the surfaces enough to prevent severe wear
seen in metal-on-poly

hydrodynamic lubrication (aka fluid film lubrication)

opposing surfaces are completely separated by a lubricant
promoted by

low surface roughness
sphericity - variation on sphericity creates localized stress points that inhibit fluid film formation
radial clearance

equatorial - radius of the head is much larger than the cup, prevents fluid lubrication
polar - radius of head is smaller than cup, poor lubrication
midpolar - optimal design, head slightly larger to prevent polar contact, but small enough to allow lubrication

ideal fluid film lubrication formed when there is a slight clearance between the socket and the head

mixed lubrication

combination of boundary and hydrodynamic lubrication
hydrodynamic when in motion, boundary when at rest or with slow motion
seen in metal-on-metal and cermic-on-ceramic

Knee Reconstruction

Introduction

Designs include

unconstrained

posterior-cruciate retaining (CR)
posterior-cruciate substituting (PS)

constrained

nonhinged
hinged

fixed versus mobile bearing

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Concepts in Prosthetic Design

Femoral rollback

definition

the posterior translation the femur with progressive flexion

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importance

improves quadriceps function and range of knee flexion by preventing posterior impingement during deep flexion

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biomechanics

rollback in the native knee is controlled by the ACL and PCL

design implications

both PCL retaining and PCL substituting designs allow for femoral rollback

PCL retaining

native PCL promotes posterior displacement of femoral condyles similar to a native knee

PCL substituting

tibial post contacts the femoral cam causing posterior displacement of the femur

Constraint

definition

the ability of a prosthesis to provide varus-valgus and flexion-extension stability in the face of ligamentous laxity or bone loss

importance

in the setting of ligamentous laxity or severe bone loss, standard cruciate-retaining or posterior-stabilized implants may not provide stability

design implications

in order of least constrained to most constrained

cruciate-retaining
posterior-stabilized (cruciate-substituting)
varus-valgus constrained (non-hinged)
rotating-hinge

Modularity

definition

the ability to augment a standard prosthesis to balance soft tissues and/or restore bone loss

options include

metal tibial baseplate with modular polyethylene insert
metal augmentation for bone loss
modular femoral and tibial stems

advantages

ability to customize implant intraoperatively

disadvantages

increased rates of osteolysis in modular components
backside polyethylene wear

micromotion between tibial baseplate and undersurface of polyethylene insert that occurs during loading

Cruciate-Retaining (CR) Design

Design $C:\Users\Sasha\Desktop\cr illustration.jpg$

minimally constrained prosthesis that depends on an intact PCL to provide stability in flexion

Indications

arthritis with minimal bone loss, minimal soft tissue laxity, and an intact PCL
varus deformity < 10 degrees
valgus deformity < 15 degrees

Radiographs

radiographs won't show box in the central portion of the femoral componentas PS knees have (see PS knee radiographs)

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Advantages

avoids tibial post-cam impingement/dislocation that may occur in PS knees
more closely resembles normal knee kinematics (controversial)
less distal femur needs to be cut than in a PS knee
improved proprioception with preservation of native PCL

Disadvantages

tight PCL may cause accelerated polyethylene wear
loose or ruptured PCL may lead to flexion instability and subluxation

Posterior Stabilized (PS) Design

Design

slightly more constrained prosthesis the sacrifices PCL
femoral component contains a cam that engages the tibial polyethylene post during flexion
polyethylene inserts are more congruent, or deeply "dished"

Indications

previous patellectomy

reduces risk of potential anteroposterior instability in setting of a weak extensor mechanism

inflammatory arthritis

inflammatory arthritis may lead to late PCL rupture

deficient or absent PCL

Radiographs

lateral radiograph will show the outline of the cam, or box, in the femoral component

$C:\Users\Sasha\Desktop\ps knee radiograph.jpg$

Advantages

easier to balance a knee with absent PCL
arguably more range of motion
easier surgical exposure

Disadvantages

cam jump

mechanism

with loose flexion gap, or in hyperextension, the cam can rotate over the post and dislocate

treatment

initial

closed reduction by performing an anterior drawer maneuver

final

revision to address loose flexion gap

tibial post polyethylene wear
patellar "clunk" syndrome

mechanism

scar tissue gets caught in box as knee moves into extension

treatment

arthroscopic versus open resection of scar tissue

additional bone is cut from distal femur to balance extension gap

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