What is an ankle sprain?

What is the definitional distinction between Grade I, II and III sprains? 

How would these various grades of injury present distinctly on examination?

 

Start with the basics. A “sprain” is an injury to a ligament (contrasted with “strain” which refers to injury to a muscle-tendon unit).

 

Technically speaking, a sprain of a ligament is a tear, which is according to the Goldilocks rule, must be classified into one of three categories:

  • Grade 1 sprain: microscopic tearing only. The ligament is not elongated and thus no laxity is detected on exam.
  • Grade 2 sprain: The ligament is partially torn, resulting in stretching that may destabilize the joint, but an end-point is reached (and may be perceived by the examiner).
  • Grade 3 sprain: A complete tear of the ligament. Note that despite the full tearing,  instability on exam may be masked by swelling or guarding.

 

So now let’s talk about ankles. Two ligaments are the primary stabilizers of the lateral ankle:

  • The anterior talofibular ligament (ATFL) (Figure 1) is the most commonly injured ligament when an ankle is sprained. The ATFL runs from the anterior aspect of the distal fibula (lateral malleolus) down and to the outer front portion of the ankle in order to connect to the neck of the talus. It stabilizes the ankle against inversion, especially when the ankle is plantar-flexed.
  • The calcaneofibular ligament (CFL) (Figure 1) is also on the lateral side of the ankle. It starts at the tip of the fibula and runs along the lateral aspect of the ankle and into the calcaneus. It too resists inversion, but more when the ankle is dorsiflexed.

Figure 1: Lateral Ankle ligaments


There is also the posterior talo-fibular ligament (PTFL) that originates from the posterior margin of the fibula and inserts into the posterior talus. The PTFL stabilizes the ankle joint and the subtalar joint. Injuries to the PTFL are rare, unless there is an ankle dislocation or marked subluxation.

 

On the medial side, the deltoid ligament is a fan shaped band of connective tissue on the medial side of the ankle (Figures 2 and 3). It runs from the medial malleolus down into the talus and calcaneus. The deeper branch of the ligament is securely fastened in the talus, while the more superficial, broader aspect runs into the calcaneus. This ligament functions to resist eversion.


Figure 2: Medial Deltoid Ligament

Figure 3: A simplified figure shows why this is called a “deltoid” (delta-shaped) ligament.
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Ankle sprains are among the most common musculoskeletal injuries. Patients typically describe an episode where they “roll their ankle,” most commonly inward, (an “inversion” sprain, Figure 4) thereby tearing the ligaments on the outside (lateral) ankle. This is contrasted with a less common "eversion" sprain where the foot rolls to the outside and the medial (deltoid) ligament is torn.

 

Patients with an ankle sprain can experience significant pain and swelling. There is usually a limp, but unlike an ankle that has been fractured, a sprained ankle will usually tolerate some weight-bearing. In severe cases, it might take 7 to 10 days for a patient to bear weight.

 

Although the phrase “it’s just a sprain” may suggest that this is always a minor injury, ankle sprains can (in rare cases) lead to significant impairment.  (CF OTHER QUESTION)


Figure 4: Ankle Inversion, the typical mechanism of injury of an ankle sprain (Image courtesy of FootEducation.com)


How would these various grades of injury present distinctly on examination?

Patients with ankle sprains typically describe a twisting episode where they invert (or less often, evert) their ankle. Pain, swelling and difficulty ambulating are common.

 

A sprained ankle often has associated redness due to the increased blood flow to this area (Figure 5). Without a history of injury, this skin appearance may suggest cellulitis (infection of the skin). Physical examination of the acutely injured ankle will reveal swelling over the outer aspect of the ankle with tenderness over the outer front (anterolateral) aspect of the ankle.


Figure 5: Ankle swelling and redness (erythema) post-ankle sprain (image courtesy of FootEducation.com)

 

In addition to pain and swelling, the following may be noted:

  • Grade 1 sprain: Mild swelling and tenderness. No objective joint instability (meaning, the examiner cannot produce any abnormal motion) because the ligament only has microscopic damage.
  • Grade 2 sprain: More pain, swelling, tenderness, and/or ecchymosis than Grade 1. Some range of motion restrictions and loss of function is usually seen. Weight bearing is painful. Mild to moderate joint instability might be found on exam.
  • Grade 3 sprain: Significant laxity on exam—if the patient is relaxed and not too edematous. That is, instability may be masked by swelling or guarding. Patients are unable to bear weight or ambulate. Paradoxically, a Grade 3 sprain may hurt less than Grade 2 when stressed on exam because ligaments are torn completely (just as a child may have more pain with a slightly loose baby tooth than with one that is almost completely free).