Bisphosphonates

Bisphosphonates are the most widely used anti-resorptive drugs in orthopaedic and metabolic bone practice. They reduce bone turnover by inhibiting osteoclasts, which slows bone loss, raises bone mineral density, and lowers fracture risk. They are first-line treatment for osteoporosis and are also used in Paget disease, hypercalcaemia of malignancy, bone metastases and multiple myeloma, and paediatric osteogenesis imperfecta.

For the exam, three threads matter and recur throughout this topic: how they work (a clean basic-science mechanism), what they prevent (fractures, with strong randomised evidence), and what can go wrong (the rare but heavily examined complications of atypical femoral fracture and osteonecrosis of the jaw, and the drug-holiday concept that follows from them).

Bisphosphonates are stable synthetic analogues of pyrophosphate, in which the central oxygen is replaced by a carbon atom (the P-C-P backbone). That carbon makes them resistant to enzymatic breakdown, and two side chains determine behaviour: one side chain (usually a hydroxyl) gives high affinity for bone mineral, while the second side chain determines anti-resorptive potency.

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How they reach and act on the osteoclast:

• After absorption, bisphosphonates rapidly bind hydroxyapatite at sites of active bone remodelling, where mineral is exposed.
• During resorption the osteoclast ingests bone mineral and, with it, the bound bisphosphonate.
• Inside the osteoclast the drug disrupts function, the ruffled border is lost, and the cell undergoes apoptosis.
• Net effect: osteoclast-mediated resorption falls, the remodelling space closes, secondary mineralisation continues, and bone mineral density rises.

Two chemical classes with different intracellular targets:

The mevalonate-pathway step is the high-yield detail: nitrogen-containing bisphosphonates block farnesyl pyrophosphate synthase, so the small GTPases that the osteoclast depends on are never prenylated (lipid-anchored to the membrane). Statins act on the same pathway higher up (HMG-CoA reductase), which is a favourite linking question.

Relative potency runs zoledronate greater than risedronate greater than ibandronate greater than alendronate greater than etidronate, reflecting both binding affinity for bone mineral and how strongly each inhibits the target enzyme.

The pharmacokinetics explain three things examiners link together — the strict oral dosing, the renal caution, and the drug holiday:

• Very low oral bioavailability — under about 1 per cent — and it is further abolished by food, calcium, and other cations (which chelate the drug). This is the pharmacological reason for the empty-stomach, plain-water, nothing-else-for-30-to-60-minutes regimen (the "WAVE" rules); poor administration is the commonest cause of treatment failure and intolerance.
• Not metabolised. Bisphosphonates are not biotransformed — there is no hepatic metabolism and no cytochrome interaction.
• Two fates for absorbed drug: roughly half is rapidly taken up by bone at sites of active remodelling, and the remainder is excreted unchanged by the kidney. Renal clearance is why zoledronate is avoided at a low eGFR and given as a slow, well-hydrated infusion.
• The plasma half-life is short, but the skeletal half-life is extremely long — years (alendronate's is estimated at up to roughly a decade) — because the drug is buried in bone and only slowly liberated during later remodelling.
• This long skeletal residence is the BASIS of the drug holiday: fracture-protection persists for a period after stopping because the bone-bound drug keeps acting, which is exactly why a planned pause is feasible — and, by contrast, why a non-bone-bound agent such as denosumab cannot be simply stopped (see below).

Atypical Femoral Fracture (AFF)

AFFs occur in the subtrochanteric region or femoral diaphysis and have a characteristic look: a transverse or short-oblique fracture line that begins at the lateral cortex, focal thickening (beaking) of that lateral cortex, a medial spike, minimal comminution, and frequent bilateral involvement. Many patients describe prodromal thigh or groin pain for weeks to months. The absolute risk is low and the fracture-prevention benefit greatly outweighs it for most patients, but the risk rises with treatment duration.

Medication-Related Osteonecrosis of the Jaw (MRONJ)

Because the rare complications (AFF and MRONJ) are linked to cumulative duration, while the fracture benefit persists for a time after stopping (bisphosphonate stays bound to bone), guidelines suggest reassessing after about 3 to 5 years of oral therapy or 3 years of intravenous therapy:

• Lower risk patients (good bone density gain, no recent fracture): consider a drug holiday and reassess every 1 to 3 years.
• Higher risk patients (previous hip or vertebral fracture, very low bone density, ongoing steroids): continue treatment, as the fracture-prevention benefit dominates.
• The holiday is a pause with planned review, not permanent cessation - bone density and fracture risk are reassessed and treatment restarted when indicated.

Examiners frequently ask you to place bisphosphonates within the wider armamentarium and to contrast them with denosumab — the comparison that most often catches candidates out.

Bisphosphonates appear across every part of the exam and daily practice. In fracture clinic and fracture liaison services they are the backbone of secondary prevention after a fragility fracture. In basic-science vivas the mevalonate-pathway mechanism is a classic ask. In trauma and arthroplasty the recognition and management of atypical femoral fractures, and the dental implications of osteonecrosis of the jaw, are common scenarios. Knowing when to start, when to pause (drug holiday), and what to warn patients about is the practical core that examiners probe.

• Osteoporosis guidelines (for example AAOS/AOA bone-health initiatives, NICE/NOGG in the UK, and endocrine society guidance) consistently place bisphosphonates as first-line pharmacological therapy for fracture prevention, with intravenous zoledronate favoured where oral agents are not tolerated or adherence is poor.
• Fracture liaison services and bone-health programmes worldwide use bisphosphonates as the backbone of secondary fracture prevention after a fragility fracture.
• The drug holiday concept is endorsed across major guidelines as a way to balance long-term fracture benefit against the duration-dependent risks of AFF and MRONJ, with high-risk patients continued on therapy.
• MRONJ guidance (AAOMS position paper) standardises the definition, staging, and the emphasis on pre-treatment dental optimisation across settings.