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DFUs, Wagner Grade 3, and Osteomyelitis: Is it only skin deep?

According to Booker (2020) with Palmetto GBA, the Medicare Administrative Contractor for several states, a Wagner Grade 3 wound is defined as a, “Deep ulcer with cellulitis or abscess formation, often with osteomyelitis” (slide 18). That definition leads to the question of “how deep is deep.” The answer is that “deep” is not very deep at all. For instance, Dr. Poonawalla (2008) pointed out that the epidermis is only about as thick as a piece of paper. She went on to say that the dermis varies in thickness, but it is thicker than the epidermis and averages from one to four millimeters.

So, that leads to the question of “just how thick is a millimeter (mm)”? The United States Mint has offered us an understandable illustration. The Mint defines the thickness of a United States quarter dollar as being 1.75 mm. Hence, the dermis is only about the thickness of two quarters. If you go beyond the dermis, you are into the subcutaneous tissue or the area “beneath the skin,” as it is the layer of tissue that underlies the skin. According to Smith (2019) the subcutaneous tissue lies just above the deep fascia.

How does this information relate to diabetic foot ulcers (DFUs) and osteomyelitis? According to Giurato et. al (2017), “Approximately 60% of diabetic foot ulcers (DFUs) are complicated by infection. In more than two-thirds of the cases, infection is the main cause for major lower limb amputation in diabetic patients with foot ulceration.” They went on to state, “Osteomyelitis is usually due to non-healing ulcers and it is associated with high risk of major amputation.” They further stated:

Diabetic foot osteomyelitis (DFO) is mostly the consequence of a soft tissue infection that

spreads into the bone, involving the cortex first and then the marrow. The possible bone

involvement should be suspected in all DFUs patients with infection clinical findings, in

chronic wounds and in case of ulcer recurrence.

And, they went on to add:

Various clinical findings can help clinicians in detecting bone infection. Two specific clinical

signs are predictive of osteomyelitis. The first is the width and depth of the foot ulcer. An

ulcer larger than 2 cm2 has a sensitivity of 56% and a specificity of 92%. Deep ulcers (>

3 mm) are more easily associated with an underlying osteomyelitis than superficial ulcers

(82% vs 33%).

A second diagnostic criterion to detect DFO is the “probe-to-bone test” (PTB). PBT is

performed probing the ulcer area with a sterile blunt probe. If the probe reaches the bone

surface the PTB is considered positive.

In conclusion, patients with a deep and infected DFU are at a higher likelihood of having osteomyelitis (bone infection) and the possible need of a lower extremity amputation than most patients. Therefore, these patients require timely and adequate treatment to prevent further complications.


Booker, S. (2020, June 8). HYPERBARIC OXYGEN THERAPY. Palmetto GBA Medicare.

Giurato, L., Meloni, M., Izzo, V., & Uccioli, L. (2017, April 15). Osteomyelitis in Diabetic Foot: A

comprehensive overview. World journal of diabetes.

Meredith, S. (2022, April 28). Coin Specifications: U.S. Mint. United States Mint.

Poonawalla, T. (2008). Dermatology. Anatomy of the Skin.

Smith, Y. (2019, February 27). What is subcutaneous tissue?. News, Medical, Life Sciences.


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