The knee is the largest joint in the body, connecting the thighbone (femur) to the shinbone (tibia) and cushioned at the front by the kneecap (patella). It is a synovial joint — a highly mobile joint lined by a fluid-filled membrane that lubricates and protects the surfaces as they move — and it plays a central role in standing, walking, running and maintaining balance.
A knee fracture is a break in one or more of the bones that form this joint, most commonly the patella, but also the lower end of the femur or the upper end of the tibia. Knee fractures can range from a small hairline crack that heals with simple immobilisation, to a severe, multi-fragment break that requires surgery to restore the joint's alignment and function. Because the knee bears the full weight of the body with every step, even a relatively minor fracture can significantly affect mobility and independence if not properly diagnosed and treated.
Types of Knee Fracture
Knee fractures are classified according to which bone is broken and the pattern of the break, both of which strongly influence treatment choice and expected recovery.
- Patella (Kneecap) Fracture: A break in the small, triangular bone at the front of the knee, usually caused by a direct blow or fall onto a bent knee, or by a sudden, forceful contraction of the thigh muscle that pulls the kneecap apart. Often makes it difficult or impossible to straighten the knee.
- Distal Femur Fracture: A break at the lower end of the thighbone, just above the knee joint. Can occur from high-energy trauma such as a road traffic accident in younger patients, or from a simple fall in older adults with weakened bone.
- Proximal Tibia (Tibial Plateau) Fracture: A break in the flat, weight-bearing top surface of the shinbone, often caused by a fall from height, a direct blow to the side of the knee, or high-impact sporting injuries. Frequently involves the joint surface itself, making precise treatment important to prevent long-term joint damage.
- Stress Fracture: A small, hairline crack that develops gradually from repetitive loading of the bone, rather than a single traumatic event — more common in athletes, military recruits, and those with weakened bone density.
- Open (Compound) Fracture: A more severe fracture in which the broken bone breaks through the skin, carrying a higher risk of infection and typically requiring urgent surgical treatment.
What Causes a Knee Fracture
Knee fractures generally result from forces greater than the bone can withstand, whether delivered suddenly through trauma or gradually through repetitive stress or weakened bone:
- Falls: Falling directly onto the knee, or landing awkwardly from a height, is one of the most common causes of patella and tibial plateau fractures, particularly in older adults.
- Direct impact or blunt trauma: A hard blow to the knee — during contact sports, a workplace accident, or the knee striking a dashboard in a motor vehicle collision — can fracture any of the bones forming the joint.
- High-energy trauma: Road traffic accidents and falls from significant height can cause more severe, complex fractures, often involving multiple bone fragments and sometimes affecting the surrounding soft tissue and blood vessels.
- Sudden muscle contraction: A rapid, forceful contraction of the thigh muscle (quadriceps) — such as when trying to catch your balance after a stumble — can pull the kneecap apart, resulting in a patella fracture without any direct blow to the knee.
- Weakened bone: Conditions that reduce bone density, such as osteoporosis, mean that even a low-energy fall or minor twist can be enough to cause a fracture, particularly in older adults.
- Repetitive stress: Stress fractures develop gradually from repeated loading of the bone without adequate recovery time, common in runners and other endurance athletes.
Symptoms of a Knee Fracture
The symptoms of a knee fracture vary with the severity and exact location of the break, but typically appear suddenly after an injury:
- Sudden, severe pain at the front, side or around the knee, worsened by any attempt to move the joint
- Rapid swelling around the knee, sometimes accompanied by visible bruising
- Difficulty or inability to straighten the knee, particularly with patella fractures
- Inability to bear weight or walk on the affected leg
- A visible deformity or abnormal shape of the knee in more severe fractures
- Tenderness to touch directly over the fractured bone
- A feeling of instability, as though the knee cannot support the leg
- In open fractures, a visible wound with bone potentially exposed, requiring emergency care
Because these symptoms can overlap with other significant knee injuries, such as a torn ligament or meniscus, prompt medical evaluation — including imaging — is important to establish an accurate diagnosis and guide appropriate treatment.
Risk Factors for a Knee Fracture
While a knee fracture can happen to anyone through sufficient trauma, certain factors increase the likelihood of injury:
- Osteoporosis and reduced bone density: Weaker bones are more likely to fracture from relatively low-energy falls, particularly in older adults and postmenopausal women.
- Participation in high-impact or contact sports: Activities such as football, rugby, skiing and cycling carry a higher risk of direct knee trauma.
- Motor vehicle and traffic accidents: A leading cause of high-energy knee fractures, particularly distal femur and tibial plateau fractures.
- Occupational hazards: Jobs involving work at height, heavy machinery, or manual labour carry a higher risk of falls and direct impact injuries to the knee.
- Older age: Age-related bone weakening and a higher likelihood of falls both contribute to increased fracture risk in older adults.
- Previous knee injury or surgery: Prior trauma or surgical hardware in the knee can alter bone strength and structure, potentially increasing the risk of future fracture.
Possible Complications of a Knee Fracture
With timely and appropriate treatment, most knee fractures heal well, but as with any significant joint injury, complications can occur, particularly with more severe or poorly managed fractures:
- Knee stiffness and reduced range of motion: A period of immobilisation, combined with swelling and scar tissue formation, can lead to lasting stiffness if rehabilitation is not started and progressed appropriately.
- Post-traumatic arthritis: Fractures that involve the joint surface, particularly tibial plateau fractures, can lead to earlier onset of arthritis in the affected knee, even after the fracture itself has healed.
- Malunion or nonunion: The bone may heal in a misaligned position (malunion) or fail to heal altogether (nonunion), both of which can affect knee function and may require further surgical correction.
- Infection: A particular risk with open fractures or following surgical fixation, requiring prompt treatment to prevent it from spreading to the bone itself.
- Nerve or blood vessel injury: Severe fractures, particularly around the knee, can occasionally damage nearby nerves or blood vessels, which may cause numbness, weakness, or reduced circulation in the lower leg.
- Chronic pain or reduced function: Some patients experience ongoing discomfort or a reduced ability to fully return to their prior level of activity, particularly after more complex fractures.
How a Knee Fracture Is Diagnosed
Diagnosis begins with a clinical assessment of the injury, followed by imaging to confirm the fracture and precisely characterise its pattern and severity.
- Clinical History and Physical Examination: A discussion of how the injury occurred, followed by an examination of the knee for swelling, deformity, tenderness, and the ability to move or bear weight on the leg.
- X-ray: The primary imaging tool for diagnosing a knee fracture, clearly showing the location, pattern and displacement of the break in most cases.
- CT (Computed Tomography) Scan: Provides a more detailed, three-dimensional view of complex fractures, particularly useful for tibial plateau fractures involving the joint surface, to guide precise surgical planning.
- MRI (Magnetic Resonance Imaging): Used selectively to assess for associated soft tissue injury, such as ligament or meniscus damage, which can occur alongside a knee fracture, particularly after high-energy trauma.
Reducing Your Risk of a Knee Fracture
While accidents cannot always be avoided, several measures can meaningfully reduce your risk of sustaining a knee fracture:
- Support bone health: Adequate calcium and vitamin D intake, along with regular weight-bearing exercise, helps maintain bone density and reduces fracture risk, particularly in older adults.
- Reduce fall risk at home: Remove trip hazards, improve lighting, and install grab rails where needed, especially for older adults or those with balance difficulties.
- Wear appropriate protective gear: Knee pads and other protective equipment during contact sports, cycling or activities with a risk of falls can help absorb impact and reduce injury severity.
- Practise safe driving habits: Wearing a seatbelt and maintaining a safe following distance reduces the risk and severity of knee injuries in the event of a collision.
- Strengthen the muscles around the knee: Good muscular support and balance can help you recover control after a stumble, reducing the risk of a fall-related fracture.
- Have bone density checked if at risk: Postmenopausal women and older adults with risk factors for osteoporosis may benefit from a bone density scan to guide preventive treatment.
