Your value as a clinician isn't just in immediate pain relief — it's in your expertise, your process, and your ability to navigate uncertainty. Bayesian reasoning provides a framework to manage expectations, make better diagnostic decisions, and update your understanding as new evidence emerges.
"You don't need to know everything. You need to be confident in showing patients you have a PROCESS that will get them where they need to go."
- Navigate diagnostic uncertainty with confidence
- Avoid anchoring on initial impressions
- Make better rebooking decisions using natural history data
- Understand chronic pain and belief change
- Interpret imaging findings with proper context
You don't need to know everything. Patients gain confidence from seeing you have a systematic process for navigating complexity. Talk through your reasoning OUT LOUD with them.
Check the Prognosis & Base Rates section for natural history data on 13 common MSK conditions and imaging base rates. The Communication Strategy section shows you how to talk through Bayesian reasoning with patients to build trust.
The initial likelihood of a hypothesis being true — e.g., disease prevalence in the population. This is your starting point before any evidence is collected.
Information that updates the prior probability — e.g., test results, clinical observations, mechanism of injury.
The updated probability of the hypothesis being true after considering new evidence.
99%
Most people think this — it's wrong.
Only 9%!
Out of 1000 people, 1 has the disease and tests positive. But 10 people without the disease also test positive (1% false positive). You're 1 of 11 positives.
With a second positive test, the probability jumps to 91%. Multiple positive tests are far more powerful than one.
- Setting accurate patient expectations
- Deciding when to rebook patients
- Identifying patients who deviate from expected trajectories
- Weighting your prior probabilities appropriately
The arithmetic average of all outcomes. Can be pulled higher or lower by outliers.
Acute LBP mean: ~6 weeks
Includes patients who recover in 2 weeks AND those who take 12–24 weeks, pulling the average higher than what most people experience.
The middle value — represents what the majority of patients actually experience.
Acute LBP median: ~3–4 weeks
Most patients report their pain "isn't really bothering me anymore" around this timeframe, notably shorter than the mean.
Most common timeframe for significant improvement
Average pulled higher — not uncommon for recovery to extend to 12+ months
- Good prognostic signs? Anchor expectations closer to median (16 weeks)
- Poor prognostic indicators? Anchor expectations closer to mean (6–8 months)
- Update continuously based on how patient tracks in first 2–4 weeks
- Start Back tool scores LOW
- Good social support and health literacy
- Positive early feedback (first 2 weeks showing good progress)
- Patient demonstrating good self-efficacy
- Start Back tool scores HIGH
- Poor prognostic indicators present
- Slow or minimal improvement in first 2 weeks
- Multiple comorbidities or complexity factors
- Most patients cluster around the median (the peak of the bell curve)
- Some patients take much longer — the tail extends right
- This tail pulls the mean higher than where most patients sit
- Your job: use the initial assessment to predict which part of the curve this patient will inhabit
Notice how low prevalence dramatically affects your posterior probability, even with highly accurate tests. This is why starting with base rate (prevalence) is diagnostically stronger than jumping straight to tests. Your clinical tests are most powerful when they update an already reasonable prior probability.
- 1Start with Base Rate — What's the prevalence of conditions in this population? This is your prior probability.
- 2Update with Subjective Information — Mechanism of injury, symptom patterns, timeline. First evidence update.
- 3Try to Disconfirm Your Leading Hypothesis — Not confirm it! This prevents anchoring bias.
- 4Clinical Tests — Use specific tests to further update probability before initial diagnosis.
Before collecting ANY piece of data, ask: "Why am I collecting this? How will it update my priors?"
Disconfirming questions:
- Was there trauma? (No = supports OA)
- Sudden onset? (No = supports OA)
- Consistent with degenerative pattern? (Yes = supports OA)
Disconfirming questions:
- Cutting/pivoting sports? (Yes = consider ACL)
- Heard a pop? (Yes = update to ACL likely)
- Immediate swelling? (Yes = ACL more likely)
- Starting with tests instead of base rate — jumping to Lachman's without considering prevalence
- Confirmation bias — only looking for evidence that supports your initial impression
- Anchoring too early — telling an 80-year-old "you have arthritis" without adequate disconfirmation
- Over-interpreting imaging — attributing symptoms to common incidental findings
- Natural history data = expected treatment response = your prior for rebooking decisions
- Base rates of imaging findings = how much weight to give "positive" findings
- Timeline expectations = when to update your diagnosis if patient isn't following expected trajectory
1–2 episodes per year is NORMAL for general population (even without chronic LBP). Chronic LBP patients have higher frequency.
- Acute LBP: Sharp drop in 3–5 days, asymptotes toward zero over 3–6 weeks
- Chronic LBP: Initial drop, then plateaus around baseline ±1–2 pain points
- Episodic flares are NORMAL — not "re-injury"
- Flare duration typically 3–7 days before returning to baseline
- Median improvement: 3–4 weeks; Mean: 6 weeks (pulled by outliers)
- Most improvement in first 6 weeks; beyond 6 weeks improvement slows
Most improvement in first 6 weeks. Expect episodic flares 1–2×/year minimum.
Rebook frequently in first 6 weeks if not following expected trajectory. After 6 weeks, longer intervals unless patient deviates. Build a flare management plan — flares don't mean treatment failure.
84% of asymptomatic people age 48 have disc degeneration on MRI. Spondylolysis (11.5% on CT) has NO significant association with LBP occurrence.
20–40% of general population experienced neck pain in the previous month.
- 45% show decreased pain/disability in first 6 weeks
- 60–80% of workers report neck pain 1 year later
- Course is recurrent/episodic — complete resolution is the exception
- For acute trauma (WAD): 3 trajectories — mild (45%), moderate (40%), severe (15%)
- Recovery occurs most rapidly in first 6–12 weeks, then slows
Most recovery in first 6–12 weeks; little improvement after 12 months.
Identify trajectory early. Severe WAD trajectory needs frequent follow-up. After 12 weeks, expect slower gains.
In those 60–65 years: 95% males and 70% females show degenerative changes.
- Pain decreases rapidly within first 2 weeks
- 75% of high school athletes return within 3 days, 95% within 10 days
- However, 5–46% still experience pain at 1–4 years
- 33–55% report instability long-term
- 50–85% report full recovery at approximately 3 years
Rapid early improvement (2 weeks), but significant minority have long-term issues.
Most can be managed with 1–2 week follow-ups initially. If pain persists beyond 4 weeks, increase follow-up frequency to identify those at risk for chronic issues.
Up to 25% of adults experienced shoulder pain in last year.
- About 50% recover completely within 6 months
- 32% continue to have more than minor symptoms at 1 year
- Pain beyond 3 months associated with poorer recovery
- Minimum 23 weeks for favourable prognosis; improvement may continue to 24 weeks
- Rapid improvement first 12 months, then plateau
Expect improvement over 23–24 weeks; reassess if not improving by 3 months.
Weekly follow-ups if pain >3 months at presentation. For acute onset, 2–3 week intervals, extending if following expected trajectory.
Asymptomatic RC tears: 8–40% partial thickness, 0–46% full thickness. In 60s: 25% have full tears. In 80s: 50% have full tears.
- Average duration 30.1 months (range 12–42 months)
- At 12–18 months, mild to moderate disability may persist
- 40–56% report ongoing pain and ROM loss up to 20 years after onset
- The belief that all cases resolve naturally is challenged by recent evidence
12–42 months; NOT always self-limiting. Manage patient expectations accordingly.
Can distinguish from rotator cuff pain within 7 days based on treatment response. If true frozen shoulder, longer intervals are acceptable. Focus on education and self-management.
1–2% per year in cutting/pivoting sports teams.
- 80% return to some form of sport after ACLR
- Only 67% return to pre-injury level
- 55% return to competitive level
- 20% sustain reinjury to either knee (90% during high-risk sports)
- Minimum RTS: 9–12 months (aligns with graft maturation)
9–12 months minimum for RTS. Manage expectations about level of return — not all will reach pre-injury performance.
Frequent early rehab (2–3×/week first 6 weeks), then 1–2×/week. Monthly check-ins from 3–9 months. Intensive RTS testing at 9–12 months.
30% of asymptomatic knees have meniscal tears (18% have degeneration).
- 88–90% follow moderate or high trajectory (gradual or early improvement)
- 10–12% have severe baseline impairments with minimal improvement over 5 years
- Majority experience early improvements, within normative data by 12 months
- Clinically relevant improvements can occur up to 24 months
- 84% of menisci remain stable over 8 years
Most improvement in first 12 months; can continue to 24 months.
If following favourable trajectory, can extend intervals. If in the 10–12% severe group, needs frequent monitoring and possible surgical consultation.
- 40% report unfavourable 12-month recovery
- Majority who recover favourably do so within first 3 months
- 71–91% report ongoing pain up to 20 years after initial diagnosis
- In adolescents: 55% still have pain at 2-year follow-up
- Improvements after 5–6 weeks intervention may plateau
If going to improve, expect it within 3 months. Many have persistent symptoms long-term.
Intense first 6–8 weeks (weekly sessions). If no improvement by 3 months, reassess diagnosis and consider comorbidities. Longer symptom duration at baseline = poorer prognosis.
- Sprinting-type injuries: median 15–23 days to RTS
- Stretching-type injuries: median 43–50 days to RTS (much longer)
- Wide variation in time to RTS
- MRI has negligible additional predictive value over history and examination
2–8 weeks depending on mechanism; stretching-type take 2–3× longer.
Weekly for first 2–3 weeks, then based on functional progression. Cannot accurately predict RTS from initial assessment — need serial monitoring.
- Patients can expect improvement between 3–12 months after treatment
- NOT expected to improve beyond 12 months
- Chronic symptoms persist in ~25% of patients at 10 years
- Wait-and-see: minimal improvement expected short-term
- 85% of athletes return to sport (level uncertain)
3–12 months; no improvement expected after 12 months. Advise against wait-and-see.
Every 2–3 weeks initially, monthly after 3 months. If no improvement by 12 months, consider other interventions.
- 90% achieve resolution at 1 year WITHOUT treatment
- Normal course: 12–18 months
- Long symptom duration does NOT affect prognosis
- Spontaneous improvement is likely even in prolonged cases
12–18 months; excellent prognosis even without treatment. Reassurance is powerful here.
Monthly intervals given high spontaneous resolution rate. Main clinical value is education and reassurance. Duration doesn't worsen prognosis — maintain hope.
- 80–90% have symptom resolution by 7–10 days
- Cognitive deficits, balance, and symptoms improve in first 2 weeks for most
- RTP often within 10 days
- Sizeable minority of high school/college athletes take >10 days
- 5–58% have persistent symptoms affecting daily function
Most recover in 7–14 days; minority have persistent issues.
Review every 2–3 days initially. Greater acute symptom severity = predictor of slower recovery. Low symptoms day 1–2 = favourable sign.
Many "abnormal" imaging findings are present in asymptomatic individuals. Always start with clinical presentation and use imaging to update your priors — not as the starting point.
You don't need to know everything. You need to show patients you have a PROCESS.
Patients gain confidence not from you claiming to have all the answers, but from seeing you have a systematic approach to navigating uncertainty. Talk through your Bayesian reasoning OUT LOUD with them.
- Be explicit about your process: "This is how I work through complex cases..."
- Acknowledge uncertainty: "I'm not 100% sure yet, which is why..."
- Show your system: "These are the checkpoints I use to know we're on track..."
- Explain your reasoning: "This piece of info makes me think... but this other piece makes me think..."
Paradoxically, admitting what you don't know while showing you have a method to find out builds MORE confidence than pretending to have all the answers.
- Patient: 41-year-old male, car retail business owner
- Chief Complaint: 3-month gradual onset left shoulder pain
- History: No acute injury, Type 2 diabetes (poorly controlled), hypertension and obesity
- Extensive shoulder trauma from semi-professional rugby; multiple previous dislocations
- Imaging: Ultrasound and X-ray show mild OA
- ROM pattern: AROM = PROM (equal active and passive)
- Order of loss: Internal rotation > External rotation > Abduction > Flexion
- Wall reach: 88cm abduction, 110cm flexion
- Key finding: Equal AROM/PROM with capsular pattern
Adhesive Capsulitis — metabolic conditions, age, trauma history, equal AROM/PROM all point here
Rotator Cuff Related Shoulder Pain — higher base rate in general population
Osteoarthritis — scan findings mild, wouldn't severely restrict ROM in this pattern
- Manual therapy (glides, PNF, isometrics)
- Exercise (broomstick drills, wall slides, theraband)
- Goal: Can we improve ROM by influencing the nervous system?
- Reasoning: If ROM significantly improves with these interventions, suggests rotator cuff pain rather than true capsular restriction
Rebooked in 7 days for crucial reassessment — diagnostic uncertainty is high, so short follow-up interval is warranted.
- 20 degrees greater abduction
- 14cm better wall reach
- External rotators significantly weak on dynamometry
- Adhesive capsulitis and poorly managed rotator cuff pain can present identically
- Don't anchor diagnosis too early — maintain openness to updating
- Communicate uncertainty honestly — explain your reasoning process
- Book early follow-ups when diagnostic uncertainty is high
- Treatment response is valuable diagnostic data
- Condition is straightforward and predictable
- Following textbook recovery pattern
- Patient is clearly a "normal responder"
- High confidence in prognosis
- E.g. Tennis elbow at 6 months (90% spontaneous resolution)
- Diagnostic uncertainty remains high
- Not responding as expected from natural history
- Need to determine high/normal/low responder status
- Gathering data points to update your priors
- E.g. LBP not improving in first 6 weeks
"Treatment outcomes serve as data points to confirm or contradict initial hypotheses."
- 0–3 or 7–10: Extremely difficult to change — requires multiple strong interventions over time
- 4–6: More receptive to new information and belief updates
- High responder: Exceeds typical improvements — may need less input
- Normal responder: Following expected trajectory — standard protocol
- Low responder: Underperforming — needs protocol adjustment
Early signs of improvement: less morning stiffness, easier movements, better wall reach. These appear before full recovery.
Outcome measures: return to sport, pain-free week, Start Back score. These confirm recovery has occurred.
Difficulty updating strong prior beliefs. Unexpected changes cause distress because they disconfirm expectations.
Approach: Graded exposure with predictable small updates. Same environment with incremental changes.
Too willing to update priors. Easily distracted by new stimuli, struggling to maintain a coherent worldview.
Approach: Help establish stronger priors through structure, routine, and filtering strategies.