Thumb Base Fractures

The Green & O'Brien classification lists a Type IV paediatric epiphyseal (physeal) base injury, but it behaves differently enough to deserve its own answer in a viva. The key anatomical fact is that, unlike the finger metacarpals, the thumb metacarpal has its physis at the PROXIMAL end (the base) - exactly where these fractures occur. So a "thumb base fracture" in a child is usually a physeal injury, and it must be described in Salter-Harris terms.

• Most common patterns are extra-articular - a metaphyseal base fracture or a Salter-Harris II (a metaphyseal Thurston-Holland fragment with the physis). These are usually managed by closed reduction and a thumb spica cast, and they remodel well because the physis is adjacent to the deformity and the mobile CMC joint compensates. Considerably more angulation is accepted than in an adult because of this remodelling potential.
• Salter-Harris III / IV fractures cross the physis into the CMC joint - the true intra-articular "paediatric Bennett equivalent." Because they are both intra-articular and physeal, displacement threatens the joint surface AND the growth plate; if displaced they need anatomic reduction and stabilisation (usually CRPP), with smooth wires avoiding repeated physeal crossing.
• The principle mirrors adults: extra-articular tolerates angulation; intra-articular demands congruity - but in a child you add the extra goal of protecting the physis from growth arrest (a malreduced SH-III/IV can form a physeal bar and cause angular deformity or shortening).

The topic rightly makes the Robert's view (true AP with the forearm hyperpronated, thumb dorsum on the plate) the key film for the volar-ulnar fragment and articular step-off. But the Robert's view is an AP, and the defining instability of a Bennett's fracture is dorsoradial - the dorsal component is exactly what an AP projection cannot show. The named companion film is the Bett's view (a dedicated true lateral of the first CMC joint), obtained with the hand pronated roughly 20-30 degrees from the fully pronated position with the beam angled, so the joint is profiled in the sagittal plane.

• Robert's (AP) answers "how big is the volar fragment and how much articular step-off?" The Bett's (lateral) answers "how much dorsal subluxation is there?" - you need both to characterise a Bennett's fracture, and a subluxation can be under-called on the AP alone.
• Always compare with the contralateral (uninjured) thumb, and read the post-reduction lateral to confirm the shaft has been brought back down onto the anchored volar fragment (dorsal translation eliminated), not just aligned on the AP.
• A true lateral is also the view that unmasks residual subluxation after casting or pinning - loss of reduction in a "healing" Bennett's is most often a recurrence of dorsal shift seen on the lateral while the AP still looks acceptable.

A swollen, painful thumb base after trauma is not always a Bennett's fracture. The differentials below are the ones examiners expect you to actively exclude.

• Does anatomic reduction prevent symptomatic arthritis? Kjaer-Petersen (1990) found residual displacement correlated with radiographic arthritis, yet Cannon (1986) found no reliable correlation between reduction accuracy and symptoms at a mean of 9.6 years. Cartilage injury at the moment of impact may matter as much as the residual step-off.
• CRPP vs ORIF for Bennett's. There is no randomised evidence. Most surgeons reserve ORIF for large, irreducible fragments and use CRPP first-line; the quality of articular reduction matters more than the implant chosen.
• Acceptable articular step-off. The widely quoted "1-2mm" threshold is derived from small retrospective series, not high-level evidence; some authors accept up to 1mm only, others tolerate 2mm.
• Trans-articular vs extra-articular pinning. Pinning the 1st MC to the trapezium crosses the joint; pinning to the 2nd MC base spares it. Both are used; no clear superiority.
• Rolando: ex-fix vs ORIF. Distraction external fixation (ligamentotaxis) gives reproducibly good function for comminuted patterns (Houshian 2013, Marsland 2012), challenging the older default of attempting plate ORIF on small fragments.
• Beak ligament dogma. Anatomic work (Bettinger 1999; Ladd/Hagert group) shows the dorsoradial ligament is also a major restraint, so the classic teaching that the AOL is the sole stabiliser is an oversimplification.

Thumb base fractures include Bennett's (2-part intra-articular), Rolando (3-part or comminuted intra-articular), and extra-articular patterns. Bennett's fracture is characterised by a small volar-ulnar fragment held by the deep anterior oblique (beak) ligament while the shaft displaces proximally, dorsally, and radially under APL and adductor pollicis pull. Treatment aims to restore articular congruity (under 1-2mm step-off) to limit CMC arthritis: CRPP is standard for Bennett's (traction + extension + pronation + abduction reduction), while Rolando fractures require ORIF for large fragments or distraction external fixation for comminuted "bag of bones" injuries. Extra-articular fractures are generally stable and managed with casting.