🤿 Commercial Diver Bone Necrosis Occupational Screening

A Career Under Pressure: Cumulative Hyperbaric Exposure

Dysbaric osteonecrosis (DON) is a chronic occupational bone disease of divers and compressed-air workers, caused not by a single dramatic decompression sickness (DCS) episode but by the slow accumulation of thousands of hours of hyperbaric exposure and repeated decompression cycles over a working lifetime.

• ~19–25%: Historic caisson worker DON (Bassoe, early East River tunnels)
• ~4–5%: RN clearance diver survey (Elliott & Harrison, 1970s tables)
• <1%: Modern computerized tables (incidence in surveyed cohorts)
• 3,000–12,000+: Typical career dive hours (over 10–30 years)

From "caisson disease" to a recognized occupational entity

Bone necrosis from compressed-air work was first described in the late 19th and early 20th centuries among caisson workers building bridge foundations and tunnels under compressed air — the same population in whom decompression sickness itself was first characterized. Bassoe's 1913 survey of tunnel and caisson workers found radiographic bone lesions in roughly a fifth to a quarter of long-serving men, decades before the underlying mechanism was understood.

As commercial and military diving expanded through the mid-20th century — offshore oil and gas construction, salvage, clearance diving, and eventually saturation diving to depths beyond 300 msw — the same lesions began appearing in divers who had never suffered a clinically obvious decompression sickness hit. This established DON as a distinct, often silent, occupational disease rather than merely a complication of overt DCS.

Early 1970s UK Royal Navy and commercial diver radiographic surveys (Elliott, Harrison, and colleagues, feeding the Medical Research Council Decompression Sickness Central Registry) found juxta-articular or shaft lesions in roughly 4–5% of career divers using the decompression tables of the era — figures that helped justify mandatory periodic bone screening.

What "cumulative exposure" actually means

Unlike acute DCS, which follows a single poorly-managed dive, DON risk correlates with lifetime dose: total hours under pressure, number of decompression cycles, maximum depths reached, and — critically — how conservative the decompression schedule was relative to true tissue nitrogen loading.

A career saturation diver may accumulate several thousand hours per year during active contract periods, each with its own decompression obligation. Over a 15–25 year career this can total many thousands of hyperbaric hours and hundreds to thousands of individual decompression episodes — each one a small, repeated opportunity for asymptomatic microbubble formation in bone.

This dose-accumulation model is why DON screening targets career divers specifically, rather than occasional recreational or short-contract divers, and why occupational health programs track cumulative hours and table conservatism, not just individual dive logs.

Why modern tables reduced — but did not eliminate — risk

The historical decline in DON incidence tracks closely with the introduction of more conservative decompression schedules, slower ascent rates, and computerized dive planning that accounts for multi-day repetitive and saturation exposure rather than single dives.

Early US Navy and Royal Navy air and mixed-gas tables from the 1950s–1970s were developed largely from acute DCS endpoints and permitted faster ascents than are now considered prudent for long-term skeletal health. Contemporary saturation and surface-supplied diving practice uses substantially slower decompression, real-time gas monitoring, and — for many contractors — mandatory periodic radiographic bone surveys as a formal risk-management control, not just a diagnostic afterthought.

Microvascular Bubble Occlusion in Bone Marrow

Bone is uniquely vulnerable to decompression stress because much of its blood supply — particularly near the growth plate remnants and joint surfaces — is end-arterial, with little or no collateral circulation. A transient blockage there is not quickly bypassed; the tissue it feeds simply dies.

• ~50–90%: Bone marrow fat content (of adult marrow volume)
• 2–4 h: Osteocyte ischemic death (onset of irreversible injury)
• 3 wks–3 mo: Radiographic lag after infarct (before visible changes)
• End-arterial: Vascular architecture (minimal collateral supply)

Three competing (and complementary) mechanistic theories

No single mechanism fully explains dysbaric osteonecrosis, and current understanding treats the following as overlapping contributors rather than mutually exclusive explanations:

• Autochthonous (in-situ) bubble formation — nitrogen dissolved in fatty, poorly-perfused bone marrow comes out of solution directly within marrow sinusoids and small vessels during ascent, forming bubbles that mechanically occlude local capillaries and end-arteries feeding osteocytes.

• Fat embolism — bubble-induced or pressure-related disruption of marrow fat cells releases triglyceride/fat globules into the marrow microcirculation, which then embolize downstream vessels, compounding the ischemic insult.

• Coagulation and platelet activation — circulating and intravascular bubbles denature plasma proteins and activate platelets and the clotting cascade at the bubble-blood interface, generating microthrombi that extend and prolong vascular occlusion well beyond the life of the bubble itself.

Bone is especially susceptible because osteocytes are metabolically active but sit in a rigid mineralized matrix supplied by a sparse, largely end-arterial network — there is little room for collateral vessels to open up and rescue tissue the way they can in muscle or skin.

Why occlusions can be transient yet still cause permanent damage

A microbubble or fat embolus does not need to persist for hours to cause permanent injury. Osteocytes and marrow elements are sensitive to even brief interruptions in oxygen and nutrient delivery; ischemia sustained for as little as a few hours can commit the affected volume of bone to necrosis, even after blood flow eventually resumes once the bubble resolves or the microthrombus is cleared.

This explains why an individual decompression episode need not produce any symptoms of classical decompression sickness (joint pain, neurological signs) to still leave behind a permanent, silent focus of dead bone — the occlusion can be too small, too peripheral, or too short-lived to trigger a clinically recognized DCS hit, yet still be large enough, in the wrong location, to infarct a segment of marrow or subchondral bone.

Site selectivity: why juxta-articular bone is different

The regions classically affected by DON — the humeral head, femoral head and neck, distal femur, and proximal tibia — share a common vascular vulnerability: their subchondral and epiphyseal blood supply arises from a small number of end-arteries with minimal anastomotic reserve, similar to the vascular anatomy implicated in other forms of avascular necrosis (e.g. steroid-induced or idiopathic AVN of the femoral head).

Diaphyseal (shaft) marrow, in contrast, is supplied by a richer network including the nutrient artery and periosteal collaterals, so infarcts there are typically better tolerated, remodel with sclerotic scarring, and rarely threaten mechanical integrity of the joint surface — the key reason shaft lesions and juxta-articular lesions carry such different clinical significance.

Silent Infarct Accumulation Across a Diving Career

The defining clinical feature of dysbaric osteonecrosis is silence. Unlike acute decompression sickness, which announces itself with pain, rash, or neurological signs within hours of a dive, the great majority of bone infarcts that accumulate over a diving career produce no symptoms whatsoever until the disease is already structurally advanced.

• ~90%+: Asymptomatic at formation (of infarcts, per registry data)
• 3: Classic juxta-articular sites (humeral & femoral heads, tibia)
• Femur, tibia, humerus: Classic shaft sites (diaphyseal marrow)
• Years: Time to symptom onset (if any) (after the causative dives)

Two anatomical patterns with two very different futures

DON lesions fall into two broad radiographic and prognostic categories that are established very early — at the moment of infarction — even though their clinical consequences only diverge much later:

• Juxta-articular (near-joint) lesions — located in the epiphysis or metaphysis immediately beneath the articular cartilage of the shoulder, hip, or knee. These threaten the mechanical integrity of the joint surface itself.

• Diaphyseal (medullary/shaft) lesions — located in the marrow cavity of the long-bone shaft, away from any joint surface. These are mechanically inconsequential in the vast majority of cases.

Because both types are equally silent when they form, the diver has no way to know, from symptoms alone, which pattern — if any — they have developed. This is precisely why periodic imaging, not symptom-triggered evaluation, is the basis of occupational screening programs.

The classic anatomical distribution

Decades of radiographic screening in commercial and naval diving populations have consistently identified the same handful of long bones as the dominant sites of dysbaric osteonecrosis:

• Proximal humerus (shoulder) — a common juxta-articular site • Proximal and distal femur (hip and knee) — the most frequent and most consequential juxta-articular sites • Proximal tibia (knee) — frequent juxta-articular and shaft involvement • Femoral, tibial, and humeral shafts — the dominant diaphyseal/medullary sites

This distribution mirrors the combination of large, fatty marrow spaces (favoring bubble/fat embolism) and relatively end-arterial epiphyseal supply in these specific bones — smaller or better-collateralized bones are rarely affected.

Why silent accumulation matters for occupational medicine

Because the disease accrues invisibly, relying on divers to self-report joint pain would catch only the subset of cases that have already progressed to subchondral collapse — by which point the joint-preserving treatment window has usually closed.

This is the core rationale for periodic radiographic surveillance as a formal control measure rather than a purely diagnostic tool: it converts an otherwise undetectable, slowly-accumulating occupational exposure into a monitorable, quantifiable risk that can inform decompression practice, dive scheduling, and — where necessary — early orthopedic referral before a diver ever experiences joint pain.

Radiographic & MRI Detection Before Symptoms

Because dysbaric osteonecrosis is silent at formation, detection depends entirely on scheduled imaging rather than clinical complaint. Commercial diving contractors and regulators — including the UK Health and Safety Executive under the Diving at Work Regulations — have long required periodic skeletal radiographic surveys for career divers meeting defined exposure thresholds.

• Every 2–3 yrs: Typical survey interval (for career commercial divers)
• ~30–50%: Radiographic detection threshold (bone mineral loss before visible)
• Days–weeks: MRI sensitivity advantage (earlier than plain film)
• Subchondral: "Snowcap" lesion sign (dense arc parallel to joint surface)

What the screening images show

Plain radiography remains the backbone of routine occupational screening because it is inexpensive, fast, and can survey the shoulders, hips, and knees of an entire dive team efficiently. Classic findings include:

• Juxta-articular sclerosis — a dense, curvilinear band of increased bone density running parallel to and just beneath the articular surface of the humeral or femoral head, often called a "snowcap" lesion for its cap-like appearance over the joint contour

• Medullary (shaft) lesions — irregular, patchy areas of increased density with serpentine or "smoke-ring" sclerotic margins in the diaphysis, usually with no overlying cortical or joint-surface involvement

• Mottled or moth-eaten medullary calcification reflecting old, remodeled infarcts of varying age

MRI, where available, is substantially more sensitive for early lesions — it can detect marrow edema and early ischemic change within days to weeks of the causative insult, long before enough bone mineral has been lost for plain film to show anything, making it valuable for confirming equivocal or early juxta-articular findings.

Plain radiographs typically only become abnormal once roughly a third to half of local bone mineral content has been lost or remodeled — meaning a "clean" X-ray does not guarantee the complete absence of very recent infarction, which is one reason serial screening at fixed intervals is used rather than a single one-off study.

How occupational screening programs are structured

Regulatory and industry-standard programs typically combine a baseline pre-employment skeletal survey with periodic follow-up imaging tied to cumulative exposure — commonly every two to three years for active career divers, or after specific triggering events such as an inadequately decompressed dive, an omitted decompression stop, or a treated case of decompression sickness.

Surveys conventionally target the shoulders, hips, and knees bilaterally, since these joints host the great majority of clinically significant juxta-articular lesions. Any new or progressive lesion prompts occupational health review, additional imaging (often MRI), and orthopedic referral where a juxta-articular lesion threatens the joint surface.

Juxta-articular vs shaft (medullary) lesions

Clinical Progression and Occupational Health Decisions

The clinical trajectory of dysbaric osteonecrosis depends almost entirely on lesion location. Juxta-articular lesions carry a real risk of progressing to joint collapse and end-stage arthritis, while diaphyseal lesions — the majority of cases — remain lifelong incidental findings that rarely affect a diver's health or career.

• Variable: Juxta-articular collapse risk (depends on lesion size & location)
• ~None: Shaft lesion clinical impact (in the large majority of cases)
• Arthroplasty: Definitive treatment (if collapsed) (joint replacement, as in AVN)
• Table conservatism: Primary prevention lever (+ exposure limitation)

From subchondral infarct to joint collapse

Where a juxta-articular infarct is large enough or positioned directly beneath the weight-bearing portion of the joint surface, the affected bone can gradually lose its ability to support normal mechanical load. Progression follows a pattern closely paralleling other causes of avascular necrosis of the femoral head:

1. A subchondral sclerotic lesion forms silently and is first identified on screening 2. Over months to years, the necrotic segment can develop a subtle subchondral fracture (a "crescent sign" on imaging) as the dead bone fails under repetitive load 3. The articular surface subsequently flattens and collapses, deforming the joint contour 4. Secondary osteoarthritis develops rapidly once the joint surface is no longer congruent

Not every juxta-articular lesion progresses this far — many remain stable indefinitely — but size, location relative to the weight-bearing surface, and continued occupational exposure all influence the risk, which is why juxta-articular findings trigger closer monitoring than shaft lesions.

When collapse and secondary osteoarthritis do occur, the definitive treatment is the same as for any end-stage avascular necrosis of a major joint: total joint arthroplasty (hip, shoulder, or knee replacement) — a major procedure with lifelong implant considerations for a diver, especially one still hoping to return to hyperbaric work.

Why shaft lesions are usually a non-event

Diaphyseal or medullary lesions, despite looking dramatic on X-ray with their patchy, "smoke-ring" calcification, sit away from any load-bearing joint surface and within a shaft that is mechanically robust even with a segment of dead marrow inside it. The overwhelming majority of divers with shaft lesions remain entirely asymptomatic for life, with the lesion discovered incidentally on routine screening and then simply followed at standard re-survey intervals.

Pathologic fracture through a diaphyseal DON lesion is rare and typically requires either an unusually large lesion or substantial additional trauma. For occupational health purposes, an isolated stable shaft lesion is generally not, by itself, a reason to restrict a diver's fitness to continue working.

Screening-driven occupational and career decisions

A confirmed or progressing juxta-articular lesion changes the occupational health conversation substantially. Depending on severity, location, and trajectory on serial imaging, medical advisors may recommend closer interval monitoring with MRI, modification of future decompression practice to reduce further cumulative exposure, restriction from further saturation or deep bounce diving, or — in advanced cases with joint involvement — permanent unfitness for further hyperbaric work.

Because table conservatism is one of the few levers a diver and their employer can actually control, occupational medicine programs pair screening with decompression-practice review: divers with early lesions or high cumulative exposure are often shifted toward more conservative profiles, longer surface intervals, and stricter adherence to no-decompression or staged-decompression limits, aiming to slow further silent accumulation even if existing lesions cannot be reversed.

The long view: a preventable, monitorable disease

Dysbaric osteonecrosis is unusual among occupational diseases in that its principal driver — decompression stress — is largely quantifiable and, within limits, controllable through dive planning. The historical decline from double-digit percentage incidence in early caisson and diving cohorts to well under one percent in contemporary, conservatively-managed populations demonstrates that the disease is substantially preventable, even if not completely eliminable, given current diving physiology.

Periodic radiographic screening does not prevent bone infarction from occurring, but it converts an otherwise invisible, cumulative occupational hazard into something that can be tracked, quantified, and acted upon — protecting the minority of divers whose lesions threaten a joint, while reassuring the majority whose lesions never will.