Soft Tissue Knee Injury
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.
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.
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
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:
· Arteriography (when a knee dislocation is strongly suspected)
· Injection of methylene blue
· Knee joint aspiration
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
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).
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.
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.
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
· 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.
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.
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.
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.
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
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.
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.
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.
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.
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.
Clinical presentation of anterior cruciate ligament injury occurs either as an acute injury or as a more chronic problem of recurrent instability.
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 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.
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.
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.
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
THA Prosthesis Design
- Biomechanics of total hip arthroplasty depends on
- Austin-Moore is credited with the design of the first hemiarthroplasty
- classic Charnley design was cemented polyethylene cups with a cemented femoral stem
- 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
Press-fit Femoral Stem
- relies on biologic fixation
- compression hoop stresses provide initial stability
- Unique complications
- intraoperative fracture
- more likely in press-fit
- typically due to underreaming
- high loosening rate when used in irradiate bone (due to lack of ingrowth)
Cemented Femoral Stem
- 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
- Metal on Polyethylene
- metal (cobalt-chrome) femoral head on polyethylene acetabular liner
- longest track record of bearing surfaces
- lowest cost
- most modularity
- 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)
- 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)
- 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
- best wear properties of all bearing surfaces
- lowest coefficient of friction of all bearing surfaces
- inert particles, no concern for cancer risk
- 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
- 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
- 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
- posterior-cruciate retaining (CR)
- posterior-cruciate substituting (PS)
- fixed versus mobile bearing
Concepts in Prosthetic Design
- Femoral rollback
- the posterior translation the femur with progressive flexion
- improves quadriceps function and range of knee flexion by preventing posterior impingement during deep flexion
- rollback in the native knee is controlled by the ACL and PCL
- design implications
- the ability of a prosthesis to provide varus-valgus and flexion-extension stability in the face of ligamentous laxity or bone loss
- 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
- posterior-stabilized (cruciate-substituting)
- varus-valgus constrained (non-hinged)
- 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
- ability to customize implant intraoperatively
- 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
- minimally constrained prosthesis that depends on an intact PCL to provide stability in flexion
- arthritis with minimal bone loss, minimal soft tissue laxity, and an intact PCL
- varus deformity < 10 degrees
- valgus deformity < 15 degrees
- radiographs won't show box in the central portion of the femoral componentas PS knees have (see PS knee radiographs)
- 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
- tight PCL may cause accelerated polyethylene wear
- loose or ruptured PCL may lead to flexion instability and subluxation
Posterior Stabilized (PS) 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"
- 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
- lateral radiograph will show the outline of the cam, or box, in the femoral component
- easier to balance a knee with absent PCL
- arguably more range of motion
- easier surgical exposure
- cam jump
- with loose flexion gap, or in hyperextension, the cam can rotate over the post and dislocate
- closed reduction by performing an anterior drawer maneuver
- revision to address loose flexion gap
- tibial post polyethylene wear
- patellar "clunk" syndrome
- scar tissue gets caught in box as knee moves into extension
- arthroscopic versus open resection of scar tissue
- additional bone is cut from distal femur to balance extension gap