The human body contains 206 bones that provide structure, protect organs, allow movement and produce blood cells. While most bones are very strong, the weakest and smallest bone in the human body is the stapes, which is located in the middle ear.
What is the Stapes?
The stapes, also known as the stirrup bone, is the innermost of three tiny bones located in the middle ear. It is the smallest bone in the entire human body, measuring just 2.8 to 3.3 mm in length. The stapes gets its name from its unique stirrup-like shape.
Along with the malleus (hammer) and incus (anvil), the stapes transmits sound vibrations from the eardrum to the inner ear. These three bones together are known as the ossicles.
Anatomy of the Stapes
The stapes has a head, two legs or crura, a base and a footplate. The head of the stapes articulates with the incus near the opening to the middle ear cavity. The two slender crura connect on either side of the head and join together at the base. The footplate forms on the opposite end of the base and sits in a small oval window in the inner ear called the oval window.
Here is a breakdown of the parts of the stapes bone:
- Head – rounded upper end that touches the incus
- Neck – narrow section below the head
- Crura – two legs or pillars that extend down
- Base – the part between the crura where they join
- Footplate – oval end that sits in the oval window
Stapes Function in Hearing
The stapes plays a vital role in the conduction of sound from the outer ear to the inner ear. Here is how it works:
- Sound waves enter the outer ear and travel down the ear canal.
- The waves hit the eardrum, causing it to vibrate.
- Vibrations pass from the eardrum to the malleus.
- The malleus moves the incus back and forth.
- The incus then pushes the stapes in and out.
- This movement of the stapes footplate transmits vibrations into the fluid of the inner ear.
- Sensory cells convert vibrations into nerve signals that travel to the brain.
- The brain interprets these signals as sound.
Without the stapes transferring vibrations to the inner ear, hearing would not be possible. Even though it is the smallest ossicle, the stapes has an enormously important job!
Why is the Stapes the Weakest Bone?
There are a few key reasons why the stapes is considered the weakest and most fragile bone in the body:
- Small size – The stapes is simply much tinier than other bones. Its diminutive size means it has less physical strength and structural integrity compared to larger bones.
- Thin crura – The two slender crura or legs of the stapes are very thin and delicate. Just 0.3 mm wide, the crura are vulnerable to damage and fractures.
- Poor blood supply – The stapes does not have a great blood supply. Minimal vascularity means it has less ability to heal if injured.
- Fixed footplate – Unlike other ossicles, the stapes footplate cannot move freely. It is fixed rigidly in the oval window, making the bone more susceptible to trauma.
- Composition – The stapes contains higher amounts of inorganic material and lower amounts of flexible collagen compared to other ear ossicles.
Due to these anatomical and structural factors, the stapes has very low fracture toughness and is the most breakable bone in the human body.
Diseases and Conditions Affecting the Stapes
Because the stapes is so fragile, it is prone to damage from various diseases, disorders and injuries. Some common conditions affecting the stapes bone include:
- Otosclerosis – Abnormal bone growth that causes the stapes footplate to stiffen and stop vibrating properly. This conducts hearing loss.
- Stapes fractures – Direct trauma to the temporal bone can fracture the delicate crura. Loud noises like explosions can also cause fractures.
- Osteogenesis imperfecta – Genetic disorder that weakens collagen and leads to frequent fractures, including stapes fractures.
- Tympanosclerosis – Scar tissue caused by middle ear infections may immobilize or fix the stapes in place.
- Congenital stapes malformations – Birth defects affecting stapes development.
When the stapes is damaged or fixed in place, conductive hearing loss occurs. Fortunately, medical procedures like stapedectomy can restore hearing by bypassing a dysfunctional stapes bone.
Stapedectomy Surgery
Stapedectomy is a surgical procedure performed to improve hearing when the stapes becomes immobilized by conditions like otosclerosis. It has been used since the 1950s and involves removing part or all of the stapes, then replacing it with a small prosthetic device.
Here are the general steps involved in a stapedectomy operation:
- An incision is made in the ear canal and the eardrum is lifted to visualize the middle ear.
- Using an operating microscope, the surgeon carefully separates the stapes from the incus.
- A small hole is drilled through the stapes footplate.
- The stapes bone is removed and replaced with a prosthesis.
- The incision is closed with stitches and packed with gelfoam.
- Over time, the prosthesis restores vibration between the incus and inner ear.
Stapedectomy has a high success rate, with over 90% of patients experiencing improved hearing after surgery. However, there are risks including infection, bleeding, facial nerve damage and continued hearing loss. Recovery involves avoiding straining, nose blowing and loud noises as the ear heals over several weeks.
Stapedectomy vs. Stapedotomy
Stapedotomy is a similar but slightly less invasive operation. Rather than removing the entire stapes, only a small hole is created in the footplate. This preserves more of the natural bone while still allowing fluid to pass through for improved hearing.
The main differences between stapedectomy and stapedotomy include:
| Stapedectomy | Stapedotomy |
|---|---|
| – Removes entire stapes bone | – Only creates small hole in footplate |
| – Prosthesis replaces stapes | – Natural stapes remains mostly intact |
| – Slightly higher risks | – Considered safer procedure |
In many cases today, stapedotomy is preferred since it is less traumatic and requires less recovery time compared to full stapedectomy surgery.
Conclusion
Weighing just a few milligrams, the stapes is the tiniest and most fragile bone in the entire human body. Its small size and thin crura render it very susceptible to damage, yet the stapes serves the vital function of conducting sound waves into the inner ear. Conditions like otosclerosis that affect stapes movement can significantly impair hearing, but innovations in stapedectomy and stapedotomy surgeries allow the stapes bone to be repaired or bypassed.
While it may be the weakest, the stapes remains one of the most important bones for hearing and ear function. Its diminutive but mighty presence in the middle ear allows us to perceive the wide range of sounds we rely on for communication, balance, enjoyment and survival.