Component Selection in THR

Stem Length, Stem Diameter, Neck Length, Medial Offset, Vertical Height

Offset: Horizontal distance from the center of rotation (COR) of the femoral head to the axis of the femur.
Modular Stems:
- Modular neck and head components allow for more variation in version and offset.
- Modular designs are beneficial for deformity or revision surgeries but introduce more corrosion and fretting points.

Head Types: Skirted or non-skirted.
Neck Shapes: Trapezoidal or cylindrical.
ROM (Range of Motion):
- Larger head-neck ratio reduces impingement and allows for a greater ROM.
- Impingement: May occur between the component and bone or between component and component (typically neck on cup).
- Impingement is minimized by larger heads and trapezoidal necks.
- Excessively large heads can lead to stress on the taper, causing fretting.
- Optimal Head Size: Studies (e.g., Livermore) suggest 28mm as the optimal head size for minimizing volumetric and linear wear.
- There is a trade-off between ROM, stability, and wear.

Goal: Reduce joint reaction force (JRF) by THR design. This was achieved by:
- Cup Medialization: Removing subchondral bone to reduce the body weight moment arm.
- Thin Polyethylene Cups: Although they reduce JRF, they face problems with wear and loosening due to increased stress on trabecular bone.

Long-Term Success: Cemented cups have excellent long-term data and are an option for elderly patients with poor bone quality.
Advantages:
- Less expensive.
- Flexible positioning, especially useful in deformities and revision cases.
- Good for revision with bone stock loss, as impaction grafting can be used.
Cementing Guidelines:
- Preserve the subchondral plate.
- Use at least a 5mm thick polyethylene to reduce stress transfer and wear.

Metallic Backing: Essential for stable, rapid fixation.
Reaming: A well-reamed hemisphere, with a cup of the same shape, maximizes contact and ensures stability.
Porous Coating: Facilitates bone ingrowth but requires perfect fit between insert and cup to avoid micromotion.
Micromotion Risk: Causes backside wear, so the fit must be perfect.

Polar Contact: Occurs if the cup is smaller than the reamed hemisphere, leading to instability due to peripheral gaps.
Equatorial Contact: Occurs if the cup fits the rim, which risks fracture and polar gaps.
Press Fit Cups: Preferentially fit the equatorial rim.
- Stabilization: Achieved by hoop stresses generated during implantation.
- Full Seating: Critical for preventing polar gapping, which can lead to loosening over time.

Purpose: Protect against instability in specific directions but may cause undesired impingement.

Used When: Instability persists even after addressing all other factors (e.g., soft tissue loss).
Disadvantages:
- Causes stress transfer to the cup-bone interface, which leads to premature loosening.
- Not recommended for high-demand patients or when long-term longevity is a goal.

Material: Trabecular metal with properties resembling trabecular bone.
Benefits: More potential for bone ingrowth.
Challenges: Expensive, and shearing of the coating can occur, though ideal for revision cases with minimal contact.

Jumbo Size Cups: Large hemispherical trabecular metal cups designed to fill large bony defects.
Custom Shaped Cups: Often trabecular metal with extensions to fill cavitatory defects, or designed for protrusio deformities.

Trabecular Metal Augments: Secure revision cups and help fill bone defects in a customized manner.
Rings and Anti-Protrusio Cages: Used with impaction grafting to support the cup, though not biologically fixed and prone to loosening.
- Best Support: Placing the cage superiorly into the ileum for immediate support during graft incorporation.

References: Canale T. “Campbell’s Operative Orthopaedics, 10th edition.” Mosby, 2002.