RHK

Background 
Based on the highly successful Dual Articular Knee1,2,3 with the addition of the rotating hinge, the RHK™ is a class III or salvage knee prosthesis that addresses most revision or oncological indications for knee reconstruction where hinged constraint is required.

The RHK™ design directly evolved from the custom DA RHK™ knee which has been implanted in over 2000 knees since 1998, of which none have been revised4 due to loosening.

The RHK™ builds on the experience from the successful custom design, further decreasing bone resections, providing an improved patella tracking and greater extensor moment arm efficiency.

Where patients require very large bone resections, a mega prosthetic or whole bone replacement, the RHK segmental implants and instruments should be used.

Design Rationale
The RHK™ combines the ultimate in stability from a hinge, but also provides controlled rotation by incorporating the patented bi-helical surface.The result is a knee which is self-aligning and self-centering. The high contact areas minimise component stress, maximising longevity.

Femur - The patella tracking has been optimised to provide the best in patella friendly engagement and tracking, whilst the extensor moment arm has been increased by 19% to improve function in what are often kinematically weak knees. Distal femoral resection has been decreased by more than one third, both minimising bone loss and increasing distal femoral stability.

Articulation - Both the femur and bearing have large contact areas, with an enhanced range of motion maintaining full contact area up to 155? of flexion. The result is decreased stress at both the articulation and on the axle mechanism, with load passing directly through the bearing.

ArCom™ polyethylene is also used to protect all other articulating surfaces around both the hinge and the yoke. ArCom™ - Is reported as having a 47% increase in wear resistance compared to traditionally manufactured polyethylene4 and 42% compared to EtO sterilised polyethylene5.

Tibia - The weight-bearing meniscal bearing has a bi-helical design which articulates on the highly polished cobalt chrome tibial tray. The unique, patent protected geometry, controls rotation providing a centralising torque under axial load, mimicking the natural screw home mechanism of the normal knee.

The anti-rotation fins on the tibia have been designed using data from radiographic studies to minimise the risk of cortical impingement in smaller tibia, maintaining high stability in the cortical shell. Tibial component thickness has been minimised with the bearing to produce the least possible tibial resection.

Clinical Indications
Indications for the RHK™ knee include:
• Significant bone loss
• Gross ligamentous deficiencies
• Oncological bone replacement - primary or metastatic
• Salvage knee arthroplasty
• Trauma
• Connective tissue disorders

References
1. Goddard N. “Revision TKR using the Dual-Articular Knee - Minimum 5 year results”. EFORT, O396, June 1999.
2. Hamanen et al. “Dual Articular Knee in demanding primary and revision replacements in patients with rheumatic diseases”.Int Orth. 26:92-97, 2002.
3. Drobny TK et al. “A two stage procedure for the treatment of the infected knee prosthesis”. Orthopade, 24:360-366 (1995).
4. Customs Implants, Biomet Ltd, Swindon UK. June 2003.
5. Clarke IC et al. 43rd ORS San Francisco, CA Feb 1997.
6. Shroeder DW, Pozorski KM. 42nd ORS Atlanta, GA, Feb 1996.