Bony and Soft Tissue Debridement Around the Knee
Derek F. Amanatullah
Michael J. Chen
Sahitya K. Denduluri
James I. Huddleston
Several conditions affect the knee that requires meticulous soft tissue and bony debridement for treatment: these include open fractures, osteomyelitis, and infected total knee arthroplasty (TKA).
The expertise of plastic surgeons is frequently needed for soft tissue management and coverage as the knee is relatively devoid of surrounding soft tissues in comparison to the muscle bound femur.
The knee is a complex synovial joint composed of two joints: the tibiofemoral and patellofemoral articulations.
The tibiofemoral articulation functions as a hinge joint allowing for flexion and extension with minimal rotation.
It transfers body weight from the femur to the tibia and can experience high joint reaction forces.
The patella is the largest sesamoid bone in the human body and transmits tensile forces from the quadriceps tendon to the patellar ligament.
By increasing the lever arm of the extensor mechanism from the center of rotation of the tibiofemoral joint, the patella reduces the amount of work the quadriceps have to perform to extend the knee.
The distal femur is trapezoidal in shape with two condyles that articulate with the two condyles of the tibial plateau, forming the medial and lateral compartments of the knee. The majority of the weight is borne through the medial compartment in a native healthy knee.
The mechanical axis of the lower extremity is characterized by a straight line that passes from the center of the hip through the center of the ankle. Ideally, this line passes through the center of the knee.
Compared to the hip, which is a ball and socket joint, the knee is a relatively incongruent and relies on both static and dynamic forces for rotational, sagittal, and coronal stability.
The dynamic stabilizers are the muscles and their tendons that cross the knee joint: these include the hamstrings, the iliotibial band, the pes anserine, the two heads of the gastrocnemius, the popliteus, and the extensor mechanism.
These muscles provide stability through compression of the joint when they are activated.
The hamstrings are composed of the biceps femoris, which crosses the posterolateral aspect of the knee to insert onto the head of the fibula, and the semimembranosus, which inserts onto the posteromedial aspect of the tibial plateau.
The semitendinosus, also a hamstring muscle, crosses the medial aspect of the knee to insert onto the anteromedial proximal tibia along with gracilis and sartorius to form the pes anserine.
The iliotibial band forms from the coalescence of the gluteus maximus and tensor fascia lata and crosses the knee laterally to insert onto Gerdy tubercle, a prominence found just below the joint line on the anterolateral tibia.
The popliteus originates from the posterior aspect of the proximal tibia and inserts on to the lateral aspect of the lateral femoral condyle.
The two heads of the gastrocnemius cross posteriorly and insert onto the medial and lateral aspects of the distal femur.
The static stabilizers include the ligaments and joint capsule.
The medial collateral ligament (MCL) extends from the medial epicondyle of the femur to the medial aspect of the proximal tibia metaphysis and resists valgus forces across the knee.
The posterolateral corner (PLC) of the knee resists varus and external rotational forces at the knee: its main components are the lateral collateral ligament, which extends from the lateral epicondyle of the femur to the proximal fibular head, the popliteus muscle, and the biceps femoris tendon.
The anterior cruciate ligament (ACL) extends from the posteromedial aspect of the lateral femoral condyle and runs anteriorly to insert in between and in front of the tibial spines to resist anterior displacement of the tibia relative to the femur.
The posterior cruciate ligament (PCL) runs from the anteromedial aspect of the medial femoral condyle to the posterior tibial sulcus to resist posterior displacement of the tibia relative to the femur.
The extensor mechanism crosses the knee joint anteriorly and inserts onto the tibial tubercle.
It is composed of the four muscle bellies of the quadriceps, the quadriceps tendon, the patella, the patellar ligament, and the medial and lateral patellofemoral retinaculum.
It is critical to normal ambulatory function.
The superficial femoral artery courses down the femur on the medial side underneath the sartorius muscle and then passes through the adductor hiatus to run posterior to the knee in the popliteal fossa where it becomes the popliteal artery.
It provides the sole blood supply to the distal extremity and, if compromised, risks viability of the leg and foot.
The main blood supply to the knee is provided from superior and inferior lateral genicular arteries, the superior and inferior medial genicular arteries, the anterior recurrent tibial artery, and the descending genicular artery.
These form an anastomotic ring around the patella.
The femoral nerve crosses the knee at the medial aspect and becomes the saphenous nerve supplying sensation to the medial leg and foot.
It gives off the infrapatellar branch as it crosses the knee, supplying sensation to the anteroinferior skin of the knee.
This nerve is sacrificed frequently during anterior midline exposures.
The sciatic nerve bifurcates just above the popliteal fossa into the tibial and common peroneal nerves.
The tibial nerve joins the popliteal artery and runs directly posterior to the knee into the leg.
The common peroneal nerve crosses laterally and runs superficially around the neck of the proximal fibula before bifurcating in the deep and superficial peroneal nerves.
The blood supply to the skin of the knee typically arises from medial to lateral.
This characteristic deserves careful attention when choosing which incision to use on a knee with multiple scars.
Typically the most lateral incision is chosen so that the potential area of skin at risk for necrosis is minimized.
Conditions that warrant bony debridement around the knee include open fractures, chronic osteomyelitis, and periprosthetic joint infections (PJI) after TKA.
Bony debridement is frequently required to prevent or eliminate bacteria capable of forming biofilms.
Bacteria can exist in either a planktonic or biofilm state.
In contrast to planktonic microorganisms that are free-floating and rapidly dividing, a biofilm is characterized by sessile microorganisms that are embedded in a matrix capable of withstanding both the host’s immune system and antibiotics.
Once bacteria colonize and adhere to the necrotic bone or implants, they can rapidly form a biofilm.
Surgical debridement and removal of any implants are the most efficacious method to eradicate infection.
Open fractures involving the patella are the most common with an incidence of 6 per million per year in the general population, followed by the distal femur (5.6 per million per year) and proximal tibia (3.8 per million per year).
Road traffic accidents are the primary mechanism.
The Gustilo classification system for open fractures was first published in 1976, modified in 1984, and arguably remains the most commonly used system in orthopedic surgery.1,2
This system organizes open fractures in order of worsening prognosis according to the degree of soft tissue injury, need for soft tissue coverage and vascular insult.
Type I: usually low energy, clean, and involve an open wound less than 1 cm in size
Type II: intermediate-energy, moderately contaminated wounds greater than 1 cm (but less than 10 cm)
Type III: farm injuries or high-energy fractures with periosteal stripping and extensively contaminated open wounds greater than 10 cm:
IIIA wounds can be managed with local wound coverage and skin graft only.
IIIB requires rotational or free flap coverage.
IIIC is associated with arterial damage.
The risk of infection has been shown to directly correlate with the fracture grade.
The true extent of underlying soft tissue damage can be appreciated only after surgical exploration and debridement in the operating room, rather than in the emergency department.
All open fractures are considered contaminated with bacteria, and antibiotics must be administered as early as possible to prevent infection.
Patzakis et al. found an infection rate of 4.7% compared to 7.4% when antibiotics were given within 3 hours of injury.3
The choice of the antibiotic depends on the degree of injury and level of contamination.
Intravenous first-generation cephalosporins, such as cefazolin, are considered first line.
For Gustilo type III open fractures, extending the coverage to include gram-negative bacteria by adding an aminoglycoside is commonplace.
Osteomyelitis can arise from hematogenous seeding, contiguous spread from adjacent tissues or joints, or direct inoculation from surgery or trauma.
Hematogenous spread of bacteria typically afflicts the metaphysis in skeletally immature patients but can also seed implants especially in the case of TKA.
Contiguous spread and direct inoculation are seen after direct contamination from open fractures or surgical procedures.
The duration of symptoms designates osteomyelitis as either acute or chronic and carries important implications with regard to biofilm formation and the need for surgical debridement.
The Cierny-Mader classification system is commonly used to categorize osteomyelitis based on the extent of bony involvement and status of the host and carries prognostic value in addition to guiding therapy.
Type I involves only the medullary canal.
Type II involves a portion of the cortex.
Type III is an infection that permeates the cortex into the canal, but the bone is axially stable.
Type IV is similarly permeative but diffuse and places the bone at risk for fracture.
The status of the host is divided into classes A, B, and C.
The A host is physiologically healthy.
The B host has compromised physiology from systemic effects of the infection or medical comorbidities.
The C host is classified as one in which the morbidity of the treatment is greater than the disease.
Sequestrum is a key pathologic feature of osteomyelitis and represents the formation of necrotic bone.
Bacteria persist in devascularized bone and evade host defenses and antimicrobial agents.
The response of the host is to generally wall of the “bony abscess” by forming sclerotic bone around the sequestrum.
This is known as the involucrum.
Infection After Total Knee Arthroplasty
Periprosthetic joint infection after TKA is a devastating condition that occurs in 0.5% to 1.9% of primary TKAs and in 8% to 10% of revision TKAs.4,5,6
The implants provide an excellent surface for bacterial adherence, biofilm formation, and facilitate the spread of the microorganisms into the surrounding bone and soft tissues.
Infections can occur in the perioperative period or years later after a previously well functioning TKA.
The longer the duration of symptoms, the greater chance of biofilm formation on the implants and surrounding bone.
Symptom duration is commonly separated into acute (3 weeks after surgery or symptom onset) and chronic (greater than 3 weeks after surgery or symptom onset).
Aggressive surgical debridement and implant removal are key interventions to treat infections with suspected biofilm formation.
Conditions that warrant soft tissue debridement around the knee include open fractures, septic arthritis, and PJI after TKA.
Soft tissue debridement is frequently required to prevent or eliminate bacteria capable of forming biofilms.
PATIENT HISTORY AND PHYSICAL FINDINGS
Any patient presenting with an open fracture may have associated injuries to the head, chest, abdomen, pelvis, spine, and extremities.
A standard evaluation according to the Advanced Trauma Life Support guidelines should be performed to evaluate for all potentially life-threatening injuries.
The injury mechanism and details are helpful as high- vs low-energy injuries predict the extent of bony and soft tissue injury.
Low-energy trauma with an open fracture may be associated with a small inconspicuous poke hole created from inside-out injury.
High-energy trauma usually creates a much larger soft tissue defect or frank degloving of the skin and subcutaneous tissues.
A detailed neurovascular exam of the lower limb involved should be done with focus on evaluating the function of the peroneal and tibial nerves, as well as the dorsalis pedis and posterior tibial pulses.
Any suspicion of vascular injury may direct appropriate testing and consultation with a vascular surgeon.
The soft tissues must be thoroughly inspected.
Any fracture with a nearby skin laceration or defect should be considered open until proven otherwise by surgical evaluation or saline challenge, utilized in evaluation of traumatic arthrotomy and to determine the presence of possible external communication with the knee joint.
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