Baxter's Nerve Entrapment: When Feet Throw a Tantrum

The inferior calcaneal nerve then hooks underneath the calcaneus through the superficial fascia at the superior border of the abductor digiti minimi. The nerve travels along the medial edge of the quadratus plantae. As it reaches the border of the abductor hallucis, it turns and passes anteriorly to the medial calcaneal tuberosity until it reaches its distal target of the abductor digiti minimi⁽²⁾. It also functions as a sensory nerve to the fat pad under the heel. Therefore, any injury or irritations result in severe heel discomfort.

Clinical Presentation

Baxter’s nerve is vulnerable to entrapment due to the course it takes (see figure 2). There are two areas where this is most likely to happen. The first is where the nerve turns laterally between the medial edge of the quadratus plantae and the thick lateral fascia of the abductor hallucis. If this becomes particularly tight, it can contact or compress the nerve, causing pain and numbness. The second point is where the nerve moves anteriorly past the calcaneal tuberosity. If the space the nerve occupies narrows due to a bone spur, muscle hypertrophy, or overpronation of the rearfoot/midfoot complex, impingement can occur.

When clinicians start their objective assessment, they look for an overpronated foot. This is a big part of the jigsaw when putting pieces together with the clues picked up during the subjective assessment. Common symptoms and assessment tests:
1.    Pain is the most common symptom and is a nagging pain right beneath the heel. Athletes often describe it as a sharp, burning, or aching sensation. This is most likely to flare up after an activity involving repeated heel strikes, such as walking, running, or other sports. Tenderness often occurs above the abductor hallucis origin, which can induce laterally radiating discomfort and paraesthesia. It is also common for athletes to report sharp radiating pain at night after exercise⁽⁴⁾.

2.    Numbness can occur if the nerve becomes compressed rather than just irritated. As a result, athletes may experience tingling or numbness in the heel area. 

3.    Swelling is another common symptom of nerve entrapment due to the irritation. This can range from mild to severe.

4.    Atrophy of the innervated muscles. It may be difficult for clinicians to identify muscle wasting, particularly of the abductor digiti minimi muscle. However, they should maintain high suspicion during the objective assessment⁽⁵⁾.

5.    Tinel sign is a common neurological test to assess nerve entrapment. When assessing the heel, clinicians can use it to rule out tarsal tunnel syndrome, where the anterior tibial branch of the deep peroneal nerve is also affected(4). For clarity, this is the same nerve as Baxter’s nerve entrapment; however, it is more proximal.

6.    Modified Phalen’s test attempts to replicate the athlete’s heel symptoms when the clinician passively inverts and plantar flexes the foot⁽⁵⁾.

7.    The Windlass test helps to rule out plantar fasciitis.  

All of these tests and symptoms, together with an excellent subjective history, will direct clinicians toward a diagnosis, but in some cases, imaging such as ultrasound or MRI may be necessary to rule out sinister pathologies or identify the cause of nerve entrapment.

Differential Diagnosis

As with all conditions, clinicians must consider alternative pathologies. A differential diagnosis creates scope for clinical reasoning when the primary treatment plan is unsuccessful. The heel is a complex anatomical region and clinicians need sound background knowledge to identify alternative causes of pain.

Heel Pain Differential Diagnoses

•    Plantar fasciitis
•    Tarsal tunnel syndrome
•    Seronegative arthritis-induced inflammation
•    Medial calcaneal neuritis
•    Heel spurs
•    Trauma
•    Fat pad atrophy
•    Calcaneal stress fractures
•    Periosteal inflammation

Management

Once clinicians think they have identified a nerve entrapment as the primary cause of pain and dysfunction, they can develop management strategies to aid athlete recovery. Many options are available, and they must identify which is best based on their assessment. Once they identify the primary cause, they must target that as the first treatment option.

There is no single correct answer, as research shows no optimal treatment strategy. Treatment options range from rest and non-steroidal anti-inflammatories to open distal tarsal tunnel release⁽⁶⁾. As a first treatment line, clinicians typically recommend rest, but, as with so many injuries, this is rarely the sole answer. Other passive modalities include massage, non-steroidal anti-inflammatory medications, and shockwave therapy. However, despite these modalities resulting in short-term relief, failure to address the underlying cause will result in the symptoms returning.

For example, if clinicians identify over-pronation as the primary cause. They must assess the entire kinetic chain to determine why the foot is taking strain. In overuse injuries, the site of pain may be the region that is taking strain due to irregular force distribution or mechanics in other areas along the kinetic chain. In over-pronation, clinicians must assess knee, hip, and lumbopelvic strength and mechanics. Addressing these deficits through a comprehensive rehabilitation program is the best chance for long-term success. As a shorter-term solution, clinicians may consider orthotics to help offload the heel and distribute weight more evenly around the foot during rehabilitation.

Furthermore, clinicians must ask questions about training volume and intensity. Tissue capacity is essential for lower limb sports; if an athlete’s training schedule is more demanding than their capacity, they are at risk for injury. They must maintain a stress vs. recovery balance. As clinicians suggest a period of active recovery, they can advocate for athletes to improve their capacity through strength training, i.e., replacing running time with strength training sessions. They must work closely with the athlete to create a personalized treatment plan that suits their needs and lifestyle.

Conclusion

Baxter’s nerve entrapment might be a sneaky little troublemaker, but with the proper guidance, clinicians can help their athletes send this foot gremlin packing. Plantar heel pain is notoriously difficult to treat, and clinicians may miss Baxter’s nerve entrapment. So, if the primary treatment plan is not working, they might want to consider this!

References

1. A&A Prac. 2020 Nov; 14(13):e01339
2. Pod Today. 2004 Nov; 17(11)
3. APMA. 2014 Jan
4. StatPearls Pub. 2023 Jan
5. Arch Clin Exp Orthop. 2023 Feb ;7:003-004
6. Op Tech in Sports Med. 2021 Sep ;29(3):150854