Residual or incompletely relieved nerve-root compression
A disc fragment, uncovertebral osteophyte, or foraminal narrowing may still compress the nerve root, especially when the compression was largely bony and in the foramen. Multifocal cervical degeneration can also mean that more than one level was symptomatic. Campbell's Operative Orthopaedics notes that cervical disc degeneration may be multifocal and other segments can become symptomatic even soon after treatment of one level (p. 2302).

Postoperative nerve-root irritation or inflammation
The nerve can remain sensitized after having been compressed, and surgical manipulation plus healing-related inflammation can produce recurrent burning, shooting, or aching arm pain without a new structural failure. This needs clinical correlation because worsening pain may also signal persistent compression.

A different or adjacent cervical level is symptomatic
An adjacent disc/foramen may have been diseased beforehand but less apparent. Adjacent-segment radiculopathy is generally more often a later issue, but pre-existing multilevel disease can present earlier. Harrison's reports subsequent radiculopathy or myelopathy at segments adjacent to a fusion at roughly 3% per year, while recognizing that this may partly reflect the natural course of cervical degenerative disease (p. 176).

Graft/cage or hardware-related problem
Examples include graft or cage subsidence, migration, plate/screw issue, or loss of foraminal height. These can reduce room for the exiting nerve root. Plain cervical radiographs, and sometimes CT or MRI, help assess this. Instrumentation-related complications are recognized after ACDF. Campbell's Operative Orthopaedics discusses anterior-instrumentation complications and the importance of stable fixation (pp. 2303-04).

Postoperative hematoma or swelling
A neck or epidural hematoma can compress structures, but this is much less typical when symptoms first recur at 3 weeks. Textbook descriptions place airway-threatening postoperative swelling/hematoma mainly within the first 1 to 36 hours, with peak swelling around 24 to 48 hours after surgery. Campbell's Operative Orthopaedics, p. 2303. Nonetheless, a new expanding neck swelling or breathing/swallowing difficulty is an emergency at any time.

Infection, including deep wound infection or epidural infection
At around 3 weeks, this deserves consideration if there is increasing neck pain, wound redness/drainage, fever, chills, pain at rest/night, or systemic illness. Infection can cause inflammatory pain and, rarely, neural compression.

Nonunion (pseudarthrosis)
Failure of fusion can eventually cause neck pain and recurrent radicular symptoms, but at only 3 weeks the fusion is not expected to be mature, so this is usually not the leading explanation for an acute recurrence. It becomes more relevant over later follow-up, especially with multilevel fusion or nicotine exposure. Campbell's Operative Orthopaedics reports higher nonunion rates with multilevel cervical fusion (pp. 2303-04).

A non-cervical cause of arm pain
Shoulder disease, brachial plexus pathology, carpal tunnel syndrome, ulnar neuropathy, or less commonly thoracic outlet syndrome may mimic cervical radiculopathy. If the pain distribution differs from before surgery, this becomes more plausible.