Knee joint structure
The knee joint is the largest articulation in the human body. Its uniqueness lies in the presence of a single articular cavity that unites three articular surfaces simultaneously. The joint is formed by:
- The area between the lateral femoral condyle and the lateral tibial condyle.
- The area between the medial femoral condyle and the corresponding tibial condyle.
- The patellofemoral joint — the articulation directly between the patella and the femur.
Role of the menisci
The articular surfaces of the condyles do not fully match each other. This incongruity is compensated by menisci — specialized fibrocartilaginous crescent-shaped disks with a wedge-shaped cross-section. Their thickened outer margin is firmly fixed to the joint capsule, while the thin inner margin remains free.
- Medial meniscus: Characterized by rigid fixation and a close connection with the joint capsule. The tendon fibers of the medial head of the quadriceps femoris muscle weave into this same medial part of the capsule.
- Lateral meniscus: Possesses greater mobility. This physiological freedom explains the fact that the outer meniscus is injured and torn significantly less often than the inner one.
- Discoid meniscus: In 2–15% of cases, a congenital anomaly occurs where the lateral meniscus covers almost the entire surface of the tibia. This shape increases the risk of degenerative changes and cartilage tears.
Extensor and ligamentous apparatus
The patella is the central link of the extensor apparatus, which also includes the quadriceps femoris muscle (and its tendon), the ligaments supporting the patella itself, and its proper ligament attached to the tibial tuberosity.
Knee stability is provided by the collateral and cruciate ligaments:
- Fibular collateral ligament (lateral): Maximum tension occurs when the lower leg is fully extended and externally rotated. When the knee flexes, it relaxes.
- Tibial collateral ligament (medial): Consists of two functional bundles. The anterior fibers tense during flexion, and the stronger posterior fibers tense during extension of the lower leg.
- Cruciate ligaments: Form reliable support for the knee joint in the sagittal plane.
Complications of knee fractures
Treatment of fractures in this area is associated with a number of risks. General complications include delayed bone healing, deformities at the fracture level, hardware failure, and purulent processes following surgical interventions.
Specific problems include:
- Nerve injury: When applying skeletal traction and passing a pin through the upper tibial metaphysis, there is a high risk of damaging the peroneal nerve. Prevention: the pin must be inserted strictly in the outside-in direction.
- Knee contracture: A typical consequence of lower third femur injuries. Prevention: correct choice of treatment method allowing joint movement from the first days, and early patient mobilization.
Briefly: answers to topical questions
What is the mechanism of a patellar fracture?
The mechanism of a patellar fracture can be direct or indirect.
- Direct mechanism — classical and most frequent, occurs when falling onto a flexed knee joint or from a direct blow to the patellar area.
- Indirect mechanism — rare, occurs due to excessive tension of the quadriceps femoris muscle (m. quadriceps femoris).
What are the types of patellar fractures?
Types of patellar fractures are classified by several criteria. According to displacement, non-displaced and displaced fractures are distinguished. Relative to the joint cavity, extra-articular and intra-articular fractures are identified. By the nature of the fracture line:
- Transverse.
- Vertical.
- Comminuted.
- Multifragmentary.
- Marginal.
- Avulsion.
- Osteochondral.
The AO/ASIF surgical classification is also applied.
What are the clinical symptoms of a patellar fracture?
Clinical symptoms of a patellar fracture include local manifestations and signs of impaired joint function.
- Local symptoms — pain, edema, deformation, and smoothing of the knee joint contours.
- Hemarthrosis — accumulation of blood in the joint, manifested by the patellar tapotement (ballottement) sign.
- Impaired function — forced extension of the limb, sharp pain upon attempted flexion.
- “Stuck heel” sign — inability to lift the straightened leg.
- Specific signs — crepitus of fragments, pathological mobility, palpable bone fragments, and soft tissue depression (diastasis).
In open fractures, a wound is observed with visualization of bone fragments and leakage of synovial fluid.
How is conservative treatment of patellar fractures performed?
Conservative treatment of patellar fractures is performed in the absence of extensor apparatus damage and articular surface deformation. It is indicated when bone fragment diastasis is no more than 2–3 mm (displacement up to 5 mm is allowed in children). The technique includes the following stages:
- Knee joint puncture — performed to remove hemarthrosis.
- Anesthesia — injection of 20.0 ml of a 2% novocaine solution into the joint.
- Immobilization — application of a plaster cast or orthosis from the gluteal fold to the ankle joint for a period of 1 month (in children, a deep posterior splint or knee immobilizer is used for 2–3 weeks). The limb is positioned in slight knee flexion of 5°.
X-ray control is mandatory after cast application.
What are the indications for surgical treatment of a patellar fracture?
Indications for surgical treatment of a patellar fracture are clinically significant fragment displacement and damage to the extensor apparatus. Surgery is strictly indicated under the following conditions:
- Fragment displacement — separation of bone fragments (diastasis) greater than 3 mm (in children — greater than 5 mm).
- Articular surface deformation — presence of a step-off displacement greater than 2 mm, as its persistence leads to the progression of post-traumatic patellofemoral osteoarthritis.
- Injury to the extensor apparatus of the lower leg requires mandatory surgical repair to restore limb function.
Class and exam questions
Why is the medial meniscus injured more often than the lateral one?
The medial meniscus is more rigidly fixed and closely associated with the joint capsule, whereas the lateral meniscus is more mobile, which protects it from tears under loads.
What is the danger of passing a skeletal traction pin through the tibia?
There is a risk of iatrogenic injury to the fibular nerve. To avoid this, the wire is always inserted from lateral to medial.
What structures comprise the extensor mechanism of the knee joint?
It comprises the quadriceps femoris with its tendon, the patella with retinacula, and the patellar ligament attaching to the tibia.
How to prevent knee joint contracture after a fracture?
Early patient mobilization is necessary, along with treatment methods that allow joint movement to begin from the very first days following the injury.
What else to study on this topic
- Biomechanics of the anterior cruciate ligament
- Classification of patellar fractures
- Surgical approaches to the patellofemoral joint
- Purulent complications following osteosynthesis
- Degenerative changes in discoid meniscus
Master the entire topic in the "Traumatology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Patellar fractures" and 812 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 35 video lessons on the subject
Other topics in the "Specialized Adult Traumatology" section
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.