Why Is My Foot Hurting on the Side? Key Causes

Side-of-the-foot pain most often traces to a problem in one of the tendons, bones, or small joints that run along the foot’s outer (lateral) or inner (medial) border. The outer edge is more commonly affected, and peroneal tendon disorders alone account for a surprisingly large share of cases that get initially mistaken for simple ankle sprains. Because so many structures sit in a tight corridor along each side of the foot, pinpointing the source usually requires more than just noting where it hurts; when the pain started, what makes it worse, and your foot’s overall shape all help narrow the list.

Peroneal Tendon Disorders

Two tendons, the peroneus longus and peroneus brevis, run behind the bony bump on the outside of your ankle and then curve along the outer edge of your foot. They help stabilize your ankle and control the way your foot rolls. Problems in these tendons are one of the most under-recognized sources of lateral foot pain. A review in EFORT Open Reviews describes peroneal tendon pathology as “an under-appreciated source of lateral hindfoot pain and dysfunction” that clinicians frequently confuse with lateral ankle ligament injuries.¹ That misdiagnosis matters because the treatment for a chronic ankle sprain and the treatment for a degenerating or split peroneal tendon can look quite different.

A related condition, painful os peroneum syndrome, involves a small extra bone (the os peroneum) embedded within the peroneus longus tendon near the bottom of the foot. When that bone fractures, shifts apart, or irritates the tendon around it, you get pain along the outer sole. The syndrome can range from an acute fracture to a slow, chronic tendon breakdown with thickening and scarring around the bone.² Not everyone has an os peroneum, so it tends to blindside people who have never heard of it.

Fifth Metatarsal Fractures

The fifth metatarsal is the long bone running along the outer edge of your midfoot, ending at the base of your little toe. Its base, the knobby bump you can feel on the outside of your foot about halfway between your heel and your pinky toe, is a common fracture site. A sudden ankle twist can yank the peroneus brevis tendon hard enough to pull a chip of bone off the base (called a tuberosity avulsion fracture). Other fractures occur slightly further along the bone, in the junction zone or the shaft, and these are more worrisome because the blood supply in that area is poor and healing can stall.

Orthopedic classification systems divide these fractures into three zones based on their exact location along the bone’s base.³ Zone 1 avulsion fractures usually heal well in a stiff-soled shoe or walking boot. Zone 2 and zone 3 fractures often take longer, and some require surgery with a screw to prevent nonunion. The practical takeaway: if you have sharp pain on the outer midfoot after twisting your ankle or landing awkwardly, an X-ray is worth getting even if you can still walk on it. Many people assume the pain is just a sprain, and a small fracture goes undetected for weeks.

Cuboid Syndrome

The cuboid is a small, roughly cube-shaped bone on the outer side of the midfoot, sitting between the heel bone and the base of the fourth and fifth metatarsals. Cuboid syndrome refers to a painful disruption in how this bone moves within its joint. The exact mechanism is still debated; it may involve a subtle shift in the bone’s alignment or irritation of the soft tissue lining inside the joint.⁴ Dancers, runners, and people who spend a lot of time on uneven ground seem more susceptible. Pain usually sits right around the outer midfoot and worsens with pushing off the toes.

Because cuboid syndrome does not show up reliably on X-rays or MRI, it is largely diagnosed by physical examination, specifically by pressing on the cuboid and testing how the midfoot moves. A manipulation technique (sometimes called a “cuboid whip”) is the traditional hands-on treatment and often provides rapid relief when the diagnosis is correct. If the pain is actually coming from a stress fracture or a peroneal tendon tear nearby, though, the same maneuver will not help, which is why getting the diagnosis right matters.

Sinus Tarsi Syndrome

Just in front of the outer ankle bone, there is a small funnel-shaped space between the heel bone (calcaneus) and the bone that sits on top of it (talus). This space is called the sinus tarsi. Sinus tarsi syndrome produces a distinctive aching or burning sensation right over that area, along with a vague feeling that the ankle is unstable. It typically develops after an ankle sprain that damages the ligaments running through the sinus tarsi, triggering inflammation and, over time, scar tissue formation in the space.⁵ The condition is usually post-traumatic and is marked by pain over the lateral opening of the sinus tarsi combined with a feeling of ankle instability.⁶

MRI studies have shown that the tissue filling the sinus tarsi in affected patients can look different depending on how long the problem has been present. In earlier stages, the changes resemble chronic inflammation; later, fibrosis (scar tissue) predominates; and in some cases, fluid-filled cysts develop.⁷ Treatment usually starts with physical therapy focused on ankle stability and proprioception. When conservative care fails, a corticosteroid injection into the sinus tarsi can confirm the diagnosis (if the pain goes away temporarily, you are probably in the right spot) and sometimes provides lasting relief.

Tailor’s Bunion and Footwear Pressure

A tailor’s bunion (bunionette) is a bony prominence on the outside of the foot at the base of the little toe, basically the mirror image of a regular bunion on the big-toe side. The name dates back to tailors who sat cross-legged on the floor, pressing the outside of the foot against hard surfaces for hours. Today it is more often aggravated by narrow or pointed shoes. The swelling over the metatarsal head causes pain that can be lateral, slightly underneath, or both, and chronic pressure from constrictive shoes is considered the main driver of discomfort.⁸ Over time, a thick callus can form over the bump, adding to the irritation.

Wider shoes or a small pad over the prominence are the first-line fixes, and they work well for most people. When the bony angle is large enough that no shoe feels comfortable, surgical correction to realign or shave down the metatarsal head becomes an option, though recovery takes several weeks.

When Pain Is on the Inner Side

Not all “side of the foot” pain is lateral. The inner (medial) border has its own set of common culprits.

The posterior tibial tendon runs from the calf, behind the inside ankle bone, and fans out across the bottom of the foot, where it is the main support for the arch. When this tendon degenerates, a condition called posterior tibial tendon dysfunction, you get medial foot pain, a sense of weakness, and a slowly collapsing arch. It is the most common cause of adult-acquired flatfoot.⁹ The condition is progressive: if the tendon keeps stretching and weakening, the arch drops further, which in turn puts more strain on the tendon, creating a cycle that can lead to significant deformity and arthritis in the surrounding joints.¹⁰ Early bracing and physical therapy can slow this process considerably, which is why recognizing it before the arch fully collapses matters.

Another medial-side cause that often surprises people is the accessory navicular, an extra piece of bone on the inside of the midfoot near the navicular bone. Roughly one in ten people have one, and most never know it. It becomes a problem when the cartilage connection between the accessory bone and the navicular gets stressed, producing inflammation and localized tenderness on the medial arch.¹¹ This is especially common in athletic adolescents, where the repetitive loading of sports aggravates the junction. Physical exam shows a visible or palpable bump on the inner midfoot, and an X-ray confirms it.¹²

How Your Arch Shape Shifts Where Pain Shows Up

Your foot’s arch height has a measurable effect on which structures take the most stress. People with high arches tend to load the outer edge of the foot more heavily, while those with flatter feet distribute force more medially. Research comparing runners with different arch heights found that high-arched runners reported more lateral injuries and more bone-related injuries, while low-arched runners had more medial injuries.¹³ A separate study of American football players found that those with high arches produced greater force on the lateral hindfoot compared to players with normal arches.¹⁴

If you have always had high arches and your lateral foot pain is a chronic, nagging issue rather than a single injury event, your foot’s mechanics are probably part of the story. Custom foot orthoses are commonly prescribed to redistribute pressure. In a large retrospective study of orthotic prescriptions from New Zealand podiatrists, lateral forefoot wedges were among the most common modifications, appearing in about one in five prescriptions, reflecting how often clinicians need to address lateral overloading.¹⁵ Flat feet, by contrast, are linked to higher rates of plantar fasciitis and shin splints.¹⁶

Ankle sprains can also ripple upward. One study found that after an inversion ankle sprain, the hip abductor muscles on the injured side were significantly weaker than on the uninjured side.¹⁷ Weak hip abductors reduce your ability to control how your leg and foot roll inward during walking and running, which can perpetuate the very lateral instability that caused the sprain in the first place. If you keep re-spraining the same ankle or have persistent outer foot pain after a sprain, hip strengthening is often part of the recovery plan.

Iselin’s Disease in Adolescents

If your teenager complains of pain right at the bump on the outside of the foot, Iselin’s disease deserves consideration. This is a traction apophysitis, essentially an overuse irritation of the growth plate at the base of the fifth metatarsal. It predominantly affects adolescents involved in regular sports and results from the peroneus brevis tendon repeatedly pulling on a growth center that has not yet fused to the rest of the bone.¹⁸ The pain location overlaps with fifth metatarsal fractures, so an X-ray is usually needed to tell the two apart. Iselin’s disease tends to resolve with activity modification and supportive footwear once the growth plate closes, which typically happens in the mid-to-late teenage years.

Nerve Entrapment

Nerves running along the foot’s borders can get trapped or compressed, producing burning, tingling, or sharp pain that does not quite match any bone or tendon problem. The sural nerve, which runs along the outer calf and the outside of the foot, is one such nerve. Because sural nerve entrapment symptoms can overlap with Achilles tendon pain or peroneal tendon issues, misdiagnosis is common. One published case involved a veteran whose sural neuropathy was treated for years as Achilles tendinosis, leading to persistent pain and severe limitations until the correct diagnosis was finally made.¹⁹

Other nerves in the area that can be entrapped include the deep peroneal nerve (producing pain or numbness in the web space between the first and second toes), the tibial nerve (tarsal tunnel syndrome, causing burning on the inner ankle and sole), and the medial plantar nerve.²⁰ A clue that you are dealing with nerve pain rather than a tendon or bone issue is the quality of the pain: burning, electrical-type sensations, numbness, or pain that worsens with direct pressure over a specific spot but does not change much with foot movement.

Systemic and Inflammatory Conditions

Sometimes lateral foot pain is not a local mechanical problem at all. Psoriatic arthritis has a striking tendency to involve the foot and ankle. In one review of 42 patients with psoriatic arthritis referred to a medical center, the foot or ankle was the most common site of joint involvement, affecting 86% of patients. More than half had the foot or ankle as the very first site of arthritis.²¹ The same review flagged a worrying pattern of diagnostic errors: eight patients had been misdiagnosed with gout or nerve compression before anyone recognized the psoriatic arthritis. Part of the problem was that clinicians failed to notice psoriatic skin lesions or did not think to connect foot symptoms with a systemic inflammatory disease.

Ultrasound studies of patients with psoriatic arthritis have shown widespread inflammation in the metatarsophalangeal joints, the joints at the base of the toes. In one study, over a third of the joints examined in psoriatic arthritis patients were painful, compared with none in the control group.²² If your foot pain is accompanied by joint stiffness that is worst in the morning, swelling in a single toe (a “sausage toe”), or any skin or nail changes suggestive of psoriasis, bringing these up with your doctor can steer the workup in the right direction.

Gout is another systemic condition notorious for foot pain. While the classic presentation is a red, hot, swollen big toe joint, gout can strike the midfoot, ankle, or outer forefoot as well. The pain tends to come on suddenly and peak within hours, which distinguishes it from most overuse injuries.

Getting a Diagnosis

The sheer number of possible causes makes imaging helpful, though the choice of imaging depends on what your clinician suspects. Standard X-rays catch fractures, arthritis, and bony prominences like accessory naviculars or large os peroneums. For soft-tissue problems, ultrasound and MRI each have strengths. High-resolution ultrasound is increasingly used as a first-line tool for ankle and foot pain because it is accessible, does not involve radiation, and allows the examiner to watch structures move in real time.²³

Ultrasound does have limits. A comparative analysis of ultrasound and MRI for detecting splits in the peroneus brevis tendon found that ultrasound had high specificity (about 85%) but limited sensitivity (around 40%), meaning it was good at confirming a split when it was seen but missed a substantial number of cases that MRI picked up.²⁴ In practice, this means a normal ultrasound does not completely rule out a peroneal tendon tear if the clinical suspicion is high, and MRI may still be warranted.

Vascular Causes Worth Knowing About

Most side-of-the-foot pain is musculoskeletal, but pain in the foot or lower leg that appears during walking and fades with rest could point to a blood-flow issue. Peripheral arterial disease most commonly presents as pain during ambulation, known as intermittent claudication, caused by narrowed arteries that cannot deliver enough blood to the working muscles.²⁵ The risk is higher if you smoke, have diabetes, or have high blood pressure. When the disease progresses to pain at rest, skin ulcers, or tissue changes, the situation becomes urgent. A simple ankle-brachial index test, which compares blood pressure in your arm to blood pressure in your ankle, can screen for the condition in a few minutes. If your foot pain consistently follows a walk-rest-walk pattern and does not clearly match a joint or tendon site, mentioning it to your doctor is a good idea.