Introduction

Your feet are made of 26 bones and many more muscles each, that functions together to make standing, walking and other movements possible. A problem in any one of these can make the simplest daily activity difficult. The talus is an irregular, saddle-shaped bone in the back of the foot. Although it is of very small size (usually around 2 inches), it plays an important role in the formation of the ankle joint together with the tibia and the fibula, the bones of the calf. Other key functions of the talus include helping in balance and weight bearing, facilitating smooth movements of the foot, and supporting other structures of the foot such as ligaments.¹ 

Bones are in constant remodelling, where new bone tissue is formed to replace aging tissue. A good blood flow to bring oxygen and other nutrients is essential for this process.² If the blood supply is disrupted due to any reason, bone regeneration will get impaired, ultimately causing the death of bone tissue and bone collapse. This process is called “avascular necrosis”.³ Some 

bones of the body are at a higher risk of avascular necrosis due to their tenuous blood supply, and the talus is one such bone.³

This article explores the causes, mechanisms and clinical features of avascular necrosis of the talus and key treatment options.

Causes and risk factors

Disruption of blood flow to the talus can occur due to traumatic or non-traumatic causes. Common underlying causes and risk factors for avascular necrosis of the talus are:⁴

1. Fractures of talar neck and body

The risk of development of avascular necrosis is dependent on the fracture type and the risk increases in those who are old, are obese, and have a history of smoking.⁵

2. Long-term use of steroids
3. Excessive alcohol use
4. Autoimmune conditions like systemic lupus erythematosus (SLE) 
5. Medical disorders such as sickle cell anemia
6. Hyperlipidemia (high cholesterol levels)
7. History of renal transplant
8. History of chemotherapy or radiotherapy
9. Blood clotting disorders such as hypofibrinolysis
10. Human immunodeficiency virus (HIV) infection

Mechanism of avascular necrosis of the talus

The talus is supplied by branches of three arteries in the foot: the posterior tibial, dorsalis pedis,and peroneal arteries. Due to its unique structure, around 60% of the talus is covered by cartilage and only the exposed 40% provide area for blood vessels to penetrate in.⁵ In addition, as the talus does not have any muscles attached to it, there is no secondary blood supply via vessels supplying the muscle as is the case for some other bones.¹ Therefore, the talus has a weak blood supply that can be disrupted relatively easily. 

Without the necessary nutrients the bone tissue would die off and bone collapse will occur. 

The initiation and progression of the abnormalities in avascular necrosis of the talus are described in five stages.⁵

• Stage 0 - Pre-clinical stage which is asymptomatic with the presence of risk factors
• Stage 1 - Pre-radiographic stage, where mild intermittent pain can occur. X-rays would not show many changes, but MRI will show bone marrow oedema
• Stage 2 - Pre-collapse stage characterised by persistent pain and mild functional impairment. X-rays show sclerosis or cystic changes, and MRI shows more extensive bone marrow oedema and necrosis.
• Stage 3 - Post-collapse stage, where structural collapse of the bone has occurred. It gives rise to severe pain and more significant difficulties in functioning. X-rays show fractures of the bony layer just beneath the cartilage and possible joint space narrowing
• Stage 4 - Severe, disabling pain occurs in the arthritic stage. X-rays show collapse of the talar head and advanced arthritis changes

Clinical features

Common symptoms of avascular necrosis of the talus include,⁴

• Pain in the foot and ankle region worsened when bearing weight - pain may be constant in more severe disease
• Swelling around the ankle joint
• Pain and restriction in ankle movements
• Joint stiffness
• Due to pain and instability 

Diagnosis

A history of fracture of the talus or other causes and predisposing factors will point towards the diagnosis.

Physical examination will show tenderness and swelling of foot and ankle region, and gait abnormalities.

Imaging studies performed to diagnose avascular necrosis of the talus include:

1. X-rays - These might not show any changes in early disease, but as the condition progresses abnormal hardening (sclerosis), calcifications, formation of crescent-shaped areas of dead bone (crescent sign), and minute fractures will be visible. More advanced disease will show collapsing and fragmentation and loss of joint space⁶
2. Magnetic Resonance Imaging (MRI) - This is the most sensitive imaging modality and can detect 95% of cases. Changes will be visible on MRI much earlier than on x-rays⁴ 

Other conditions that may result in similar clinical features include infection, stress fractures, primary osteoarthritis, and abnormal growths (neoplasms).³ 

Treatment options

Management of this condition depends on the severity of it (i.e. the stage) and whether it has occurred due to trauma or not. Generally, for early-stage disease (stages 1 and 2), conservative and joint-sparing treatment options are utilised. For more advanced stages, joint-sacrificing surgeries may be necessary.⁵

Conservative management

Aims of conservative management include management of pain, maintaining joint mobility and helping revascularisation of the bone. Main conservative management options are,⁴

• Pain management - Drugs like NSAIDs are used due to their analgesic and anti-inflammatory effect
• Non-weight bearing - This allows the bone just beneath the cartilage to redevelop good blood supply and restore the structure of the bone. This method is only suitable for stages 1 and 2
• Physical therapy - Range of motion exercises and strengthening exercises can help maintain joint function and strength.
• Orthotics - Braces and insoles are used to support the foot and reduce the pain
• Bisphosphonates - These drugs prevent bone loss and maintain bone density

Joint-sparing treatment⁷

• Core decompression - Small stainless steel wires (k wires) are drilled into the bone and necrotic bone is removed to reduce the pressure within it and stimulate the blood flow
• Bone grafting - Healthy bone tissue taken from other sites (grafts) such as the femoral head are transplanted to the affected area

Joint-sacrificing surgical procedures⁷

• Arthrodesis - Dead part of talus is resected and other bones of the calf are used to fuse the ankle joint
• Replacement surgery - Total or partial talar replacement, total ankle replacement are attempted in those who have severe disease with bone collapse

Prognosis

How well a person does after avascular necrosis of talus is dependent on the cause for the disease, the severity, and the amount of bone involved etc. The earlier it is detected, the better the outcome is. Severe disease can cause permanent bone collapse, ankle dysfunction, arthritis and chronic pain.⁴

Preventive strategies

Avascular necrosis of the talus if not treated early, can have devastating outcomes which emphasizes the importance of prevention. A few strategies to prevent the condition include,

1. Avoiding or reducing known risk factors like excessive alcohol use, smoking, and long term steroid use when alternative treatment options are available.
2. Managing medical conditions such as lupus, sickle cell anemia, diabetes etc. to minimize the risk
3. Using suitable footwear and avoidance of high-risk activities to minimize the occurrence of ankle injuries
4. Early and appropriate treatment of fractures of the talus. Follow up of those with talar fractures for at least 3 months to detect the development of avascular necrosis early⁴

In addition, early detection of the condition is paramount, as symptoms develop gradually, they may be missed till the very advanced stages. Timely interventions prevent permanent disability, need for more invasive surgical procedures and minimize pain. 

Future directions

Emergence of novel pharmacological agents and regenerative medical options is changing the way avascular necrosis of talus is treated.⁴

• Platelet-rich plasma (PRP) - PRP is obtained by extraction and concentration of platelets from the individual’s blood and is injected into the affected joint. The presence of high concentrations of growth factors stimulates tissue repair and regeneration⁸ 
• Stem cell therapy - Stem cells have a unique ability to differentiate and form various types of cells, including bone cells. Introduction of stem cells to necrotic areas of bone can trigger the body's natural healing processes and form new bone tissue⁸ 
• Angiogenesis modulators - These drugs promote the formation of new blood vessels to supply the affected areas of bone. 
• Bone-targeted therapies - These drugs can directly modulate the metabolism in bone tissue

Summary

• Platelet-rich plasma, stem cell therapy and angiogenesis modulators are emerging as promising treatment options
• Talus is a small bone in the foot responsible for weight bearing, balance, and smooth movements
• Avascular necrosis occurs when the blood supply to a bone is disrupted which results in death of bone tissue due to lack of oxygen and other nutrients
• Owing to its weak blood supply, the talus is at a higher risk of avascular necrosis
• Causes and risk factors for avascular necrosis of the talus include fractures or dislocations of the talus, long-term steroid use, excessive alcohol use, blood disorders with increased tendency to form blood clots, certain medical conditions like systemic lupus erythematosus, sickle cell anemia etc.
• Avascular necrosis of the talus can manifest as pain and swelling of the foot and ankle, restriction in movements at the ankle joint, and limping
• Early disease can be asymptomatic with advanced stages resulting in bone collapse, permanent joint destruction, joint stiffness and loss of function
• Imaging studies such as x-rays and MRI scans are done to diagnose the condition
• Early disease can be managed conservatively with pain killers, physical therapy, and activity modification to minimize weight bearing
• Advanced disease stages are treated with core decompression, bone grafting, bone fusion and talar or ankle replacement.
• Early detection of the condition is important to prevent permanent disability
• Avoiding known risk factors, optimally managing medical conditions, using proper footwear to prevent trauma, and early and appropriate treatment of talar fractures can prevent the development of avascular necrosis of the talus