Plantar Fasciitis Diagnosis
Baxter’s Entrapment is a cause of heel pain like Plantar Fasciitis. About 20% of heel pain cases are caused by Baxter’s Entrapment. Clinical picture of Baxter’s Entrapment is virtually identical to Plantar Fasciitis. So, it is frequently misdiagnosed.
Plantar Fasciitis or Baxter’s Entrapment ?
Some cases of Plantar Fasciitis may actually be Baxter’s Entrapment.
In fact, the clinical picture may of Baxter’s Entrapment and Plantar Fasciitis can be virtually identical.
Baxter’s neuropathy may account for up to 20% of heel pain cases,1,2 although it is frequently misdiagnosed. If it is misdiagnosed as Plantar Fasciitis then traditional Plantar Fasciitis treatments could be ineffective, or actually make the condition worse.
What is Baxter’s Entrapment or Baxter’s Neuropathy?
Baxter’s Entrapment is an entrapment (or compression) of the Inferior Calcaneal Nerve just under the base of the arch of the foot. The Inferior Calcaneal Nerve is the first branch of the Lateral Plantar Nerve on the bottom surface of the foot. The nerve is also sometimes called Baxter’s nerve, named after the first physician to describe this nerve entrapment as a specific cause of foot pain.
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It is difficult to identify compression of the Inferior Calcaneal nerve with physical examination. Frequently, it is painful to feel along the arch on the bottom surface of the foot. In some cases it will be especially painful to feel along the outer aspect of the bottom of the foot where the Abductor Digiti Minimi muscle is located. It may also be painful along the inner arch where the nerve is likely to be entrapped.
Ultrasound Guided Local Anesthetic Injection
It is very difficult to identify the clinical indicators of inferior calcaneal nerve compression through physical examination, so this nerve compression is often missed unless an ultrasound guided local anesthetic diagnostic test or MRI is performed. A local anesthetic agent (such as lodocaine) is injected near the origin of the Medial Calcaneal Nerve using ultrasound guidance to increase accuracy. If the trial injection results in a significant decrease in pain, then this indicates that Baxter’s Entrapment is likely to be present.
MRI has been shown to be valuable in demonstrating muscular changes associated with denervation. When motor fibers to the abductor digiti minimi muscle are damaged, the muscle will atrophy and fatty edema can build up in the muscle. The fatty infiltration of muscle is visible on the MRI. MRI is also helpful in excluding alternative diagnoses (fracture, neoplasia, fasciitis).
Ultrasound guided radiofrequency ablation is a sophisticated, minimally invasive procedure that is associated with high patient satisfaction scores and long term pain relief. It is a procedure that uses high frequency sound waves to heat the sensory nerve that is affected in Baxters entrapment to 90 degrees Celsius. At this temperature the heat breaks down proteins preventing the nerve fibers from transmitting pain. The nerve is destroyed, and the pain from that area is not transmitted any further. In addition, radiofrequency ablation may cause the creation of new blood vessels speeding up the healing process. The procedure is minimally invasive and at our center these procedures are performed under local anesthetic with ultrasound guidance to ensure correct positioning.
Radiofrequency ablation is a mostly painless procedure and you are likely to only feel pain when the local anesthetic is injected into the foot. On the day of the procedure, the skin is anesthetized with local anesthetic. After it has taken effect, a very small puncture is made over the painful area. The radiofrequency needle is introduced and an ultrasound is used to guide the positioning of the needle. The position of the needle is then rechecked and refined using a nerve stimulator. Once the needle is in the correct position, an electrode is placed through the needle and the tip of the electrode is heated to about 90 degrees Celsius for 90 seconds and repeated according to protocol. The site is covered with a bandage and patient is advised to reduce activity, ice and elevate the foot for the remainder of the day. The bandage can be removed the following day and patient can cover the area with a regular Band-Aid. You should keep the site clean and dry for at least 24 hours. Normal activity can be resumed within one or two days of the procedure and any pain that occurs is usually managed with an NSAID or Tylenol.
What to expect after Radiofrequency Ablation
Side effects are not generally seen with this procedure, but a few are a possibility. Infections are rare, as is abscess formation at the puncture site. You may develop bruising that can be painful. Numbness or a lack of feeling in around the incision area can also happen, but this side effect is rare.
You may experience relief from this procedure within a week, but often it takes a longer period of time to heal completely. We recommend continued heel pads or arch supports as well as strectching for about 2 months after the procedure. You should be able to return to sports activities such as jogging one month after the treatment. If there is still pain at four to six weeks, the procedure can be performed again. If none of these procedures affect the pain, it may be time to look at other invasive procedures. However, most patients find that after two radiofrequency ablations their pain has decreased significantly.
The Posterior Tibial Nerve divides into two primary branches, the Medial and the Lateral Plantar Nerves, in the tarsal tunnel as it passes around the medial side of the ankle. Both branches then curve around the medial ankle and course along the bottom surface of the foot.
The Medial Plantar Nerve travels anterior to the Lateral Plantar Nerve, carrying sensory information from the medial two thirds of the plantar foot, and motor innervation to the flexor digitorum brevis, abductor hallucis, flexor hallucis brevis, and first lumbrical.
The Lateral Plantar Nerve carries sensory information from the lateral plantar aspect of the foot, 5th toe, and lateral half of the 4th toe. Motor innervation involves all the remaining foot muscles, not innervated by the Medial Plantar Nerve.
The first branch that splits off from the Lateral Plantar Nerve is known as the Inferior Calcaneal Nerve or Baxter's Nerve. It is a mixed sensory and motor nerve, providing motor innervation to the abductor digiti minimi muscle. Weakness of this muscle may be present but is difficult to detect clinically. The Inferior Calcaneal Nerve originates from the Lateral Plantar Nerve at various levels beneath the deep fascia of the abductor hallucis muscle. In most people, the Inferior Calcaneal Nerve branch splits off from the Lateral Plantar Nerve on the underside of the foot. However, in some cases the Inferior Calcaneal Nerve may actually branch off more proximally, even as far up as the tarsal tunnel on the medial ankle. Rarely, the nerve can originate directly from the Posterior Tibial Nerve. The nerve courses vertically between the abductor hallucis and quadratus plantae muscles, then makes a sharp 90 degree horizontal turn, coursing laterally beneath the calcaneus to innervate the abductor digiti minimi muscle.
Where and how a nerve splits is relevant because of the compressive loads on the nerve which may lead to nerve injury. Another important factor is the sharp right angle turn the nerve takes from the medial side of the foot to the plantar surface.
The Inferior Calcaneal Nerve supplies motor innervation to the abductor digiti minimi muscle, occasionally to the flexor digitorum brevis and lateral half of the quadratus plantae. Sensory information is carried from the calcaneal periosteum, long plantar ligament, and adjacent vessels.
Nerve entrapment can occur in 1 of 2 locations:
- Between the deep fascia of the Abductor Hallucis muscle and the the medial plantar margin of Quadratus Plantae muscle; or,
- Along the anterior aspect of the Medial Calcaneal Tuberosity where the nerve can be compressed against the bone or bone spurs that may have developed along the Anterior Calcaneus bone.
- Chundru U, Liebeskind A, Seidelmann F, Fogel J, Franklin P, Beltran J. Plantar fasciitis and calcaneal spur formation are associated with abductor digiti minimi atrophy on MRI of the foot. Skeletal Radiol. 2008;37(6):505-510.
- Pecina M, Markiewitz A, Krmpotic-Nemanic J. Tunnel Syndromes: Peripheral Nerve Compression Syndromes. Boca Raton: CRC Press; 2001.
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