Tramadol (Tramacet) induced tremor: The patient is on Tramacet around the clock for pain management. Tramadol is known to cause tremors as a side effect, especially at high doses or with prolonged use. The temporal relationship between the initiation of Tramacet and the onset of tremor supports this diagnosis.

Anxiety or stress-related tremor: Recent surgery, such as knee replacement, can be a significant source of stress and anxiety, which may manifest as a whole body fine tremor.
Medication withdrawal or interaction: Although Tramacet is the mentioned medication, the patient might be on other medications that could interact with Tramacet or have withdrawal effects that cause tremors.
Essential tremor: This is a common movement disorder that can start at any age, including in older adults. The fine tremor could be an initial presentation or exacerbation of an underlying essential tremor.

Thyroid storm or hyperthyroidism: Although less common, thyroid disorders can cause tremors. Missing a diagnosis of hyperthyroidism or thyroid storm could be life-threatening.
Serotonin syndrome: Given the patient's use of Tramacet (which can increase serotonin levels), there is a risk, although small, of serotonin syndrome, especially if other serotonergic medications are being used concurrently.
Infection or sepsis: Post-surgical infections can present with systemic symptoms, including tremors. Early recognition and treatment of sepsis are crucial for survival.

Wilson's disease: A rare genetic disorder that leads to copper accumulation in the body, causing neurological symptoms, including tremors. It's rare but should be considered in younger patients or those with a family history.
Other movement disorders: Such as dystonic tremor or parkinsonian tremor, which are less likely given the acute onset in relation to surgery and medication but should be considered if the tremor pattern is atypical or does not respond to expected treatments.