From $n=2$ textbooks, the results are equivocal.
Moore et al. (2013)
- There is no mention of an extensor expansion associated with the pollex.
- EPL and EPB are listed as inserting on the distal and proximal phalanges, respectively.
Rohen et al. (2010) (p. 394)
- "Extensor expansion of extensor of thumb" The label points to the middle of the diaphysis of the proximal phalanx. It's odd that extensor is singular here, given that there are two extensors.
However, Abdel-Hamid et al. (2013) present data from dissection of 95 limbs.
Extensor pollicis longus:
"EPL tendon was attached to both proximal (PP) and distal (DP) phalanx through the extensor expansion in 94.7% of specimens (Figs. 1A, B; 2A). In 5.3% of upper limbs, the EPL tendon was attached to the DP through its extensor expansion (Fig. 2B; Table 2)."
Extensor pollicis brevis:
In 55.8% of upper limbs, [EPB] was attached distally to the base of the [proximal phalanx] of the thumb (Fig. 1B). While in 41%, EPB tendon was attached to the extensor expansion of the thumb at the level of the base of [proximal phalanx] (Fig. 2A). However, in 3.2% of specimens its tendon was inserted into the extensor expansion of the thumb at the level of the base of [distal phalanx]."
And the answer appears to still be equivocal.
Abdel-Hamid GA, El-Beshbishy RA, Abdel Aal IH. 2013. Anatomical variations of the hand extensors. Folia Morphol 72:249–257.