In cementless total hip arthroplasty, initial stability of the acetabular cup depends on 'press-fit.' The recommended under-reaming practice to achieve adequate press-fit is:

• A Ream 1–2 mm under cup outer diameter ✓
• B Ream 4 mm under cup outer diameter
• C Ream equal to cup outer diameter (no press fit)
• D Ream 5 mm over cup outer diameter (over-reaming)

Explanation

For cementless acetabular cups, under-reaming by 1–2 mm relative to the cup outer diameter creates a 'press-fit,' achieving initial mechanical stability through interference fit between the porous-coated cup and the host acetabular bone. This is sufficient to allow osseointegration (biological fixation) while avoiding fracture of the acetabular wall. Under-reaming by >2 mm risks acetabular fracture or cup proud placement; no under-reaming (line-to-line) reduces initial stability. Standard teaching for NEET: under-ream by 1–2 mm for press-fit acetabular component.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.