DynaMoM hip resurfacing device (Tornier) by J Girard, MD, MsC.

DynaMoM hip resurfacing device was introduced in France in 2008 and was first implanted at Lille Universitary Hospital by J. Girard, MD. Rationale for design The rational for the design was made according to clearance, the rugosity, the head diameter, the sphericity and the alloy process. DynaMoM design-included the extent of cement fixation, optimization of implant position, minimization of stress-shielding, geometry options, metallurgy, examination of in vivo cup deformation… Cup design

Jump factor is defined by the distance to cross before head dislocation. In hip resurfacing, mainly design shows a 66m difference between cup and head diameter.

So, if you choose a hemispherical cup design truncated with 3mm, jump factor is 16mm with a 38mm head diameter and 27mm with a 60mm head diameter. With a conventional cup design (like a polyethylene insert) with a 22.22mm head diameter, jump factor is 11mm. So an internal truncated hemispherical cup appears to improve jump factor. For the DynaMoM cup design, we choose a minimal 61° of total half-clearance in the severe configuration of a resurfacing implant with a head-neck ratio of 1,6. This 61° could be compared with the 58° of a Durom cup or a 64° of the Conserve + cup.

According to the cup size, internal sphere was truncated by a 8.4% of the diameter with a total clearance of 124°. The cup is a flattened truncated hemisphere (165° variable over size range) and cup thickness is 6mm (Fig 1 and 2). There is a truncated inlay hemisphere which led to an arc of motion of 122°.

Cup sizes

Cup sizes available ranges from 44 to 62mm with a 2mm increment. There is no dysplasia cup design. Cup press fit was equatorial on 2/3 of the cup and on exact fit for the dome area. The cup press fit was variable according to cup size. So, press fit cup is 1.5mm for cup ranges from 44 to 48mm; 1.75mm from 50 to 56mm and 2mm for cup larger than 56mm.

Porous coating

Porous coating cup design is enhanced by hydroxyapatite (Fig 1). For others manufacturers, titanium vacuum plasma sprays or Co–Cr beads are currently used with or without HA. Titanium is considered to be more biocompatible than Co–Cr. So DynaMoM cup has a double coating with one layer of Titanium plasma spray and a second layer of hydroxyapatite. This coating is associated with reliable bone in-growth ensuring long-term secondary fixation. To prevent eventual third body wear, hydroxyapatite was not present on the equatorial area (Fig 2).

Bearing concept

It’s well known that high carbon level is a relevant factor (more than 0.2%). On the other hand, a very precise alloy composition is mandatory to dramatically reduce wear (60% of Co, 30% of Chromium and 6% of Molbyden). Whilst all manufacturers now use high carbon containing Co–Cr alloy, the processing of the alloy differs (forged or cast alloy, post-casting heat treatments…).

In the same way, a forged alloy led to better wear resistance, a reducing surface roughness, an enhancing lubrication. Moreover, a forged alloy versus a cast alloy improves the elastic resistance (760MPa vs 450MPa). This property is quite interesting to limit cup deformation during impaction and to reduce cup thickness without increase deformation possibility. Surface roughness defined by lambda coefficient (film thickness ratio) enhanced lubrication type. With high roughness an then low lambda coefficients, no fluid film lubrification could be exist. So a forged allow with a high carbon level appears to be the only real solution for hip resurfacing bearing design. A low clearance increases the potential for fluid film lubrication (ie low wear and very low friction).

On the other hand, a small clearance led to cup clamping with high risk of cup deformation under load. In contrast, a great clearance not generate fluid film lubrication, enhanced low contact area (edge loading) and result to wear. A clearance of 100–150 mm appears to be the optimal compromise for large-diameter metal-on-metal articulations (resurfacing or arthroplasty). The bearing DynaMoM surface metallurgy is a forged alloy (ISO 5832-12 or ASTM F1537-00). The percentage of Carbon, Cobalt, Chromium and Molybdenum was respectively, 0.15 to 0.25%; 63%; 20-30% and 4-8%. Tribologic results were assessed at EndoLab Institute and noticed a wear rate inferior to 0,5mg per Million Cycle. Diametric clearance is constant (130 µm) and roughness is less than 0.02µm. There still exist a constant equatorial rigidity (=600 daN). Rotation centre excentration

In fact, the gap between internal and external spheres can generate a loosening moment. So the rotation centre excentration have to be minimized. On the other hand, the acetabular bone stock preservation is a necessary fact during hip resurfacing surgery. So a perfect design led to minimize the difference between the prosthetic head diameter and the external cup diameter. For the DynaMoM cup design, key features were: – a good bone stock preservation – a small rotation centre excentration – a small loosening moment on the cup side – a cup rigidity quite sufficient to prevent cup deformity (min 600 daN) – a progressive rotation centre excentration according to cup size (0.6 to 1.8mm) – a minimal cup thickness of 3mm

Specific cup design shows an excentration of the rotation centre according to the cup size (0.6 to 1.8mm). During impaction, the equatorial deformation is very low (less than 30microns). Cementation

It’s now well known that an important cement mantle induces a high cement penetration into the femoral head. So cement mantle (defined by diametrical difference between the implant and the corresponding reamer) have to be inferior to 3mm. Moreover, cement mantle thickness is correlated with a head necrosis risk by potential thermal lesions and with excessive penetration of cement into the cancellous bone of the femoral head. Excessive cementing may have biomechanical consequences in terms of decreased bone-loading in the proximal aspect of the femur. So, a low cement mantle was designed (less than 0.5mm) to improve femoral osseous stock. Some grooves were designed on the internal side of the femoral implant to reduce the cement pressurization.

The cement mantle was very low (less than 0.5mm) and there are 3 stems diameter according to femoral size (Fig 3). To reduce femoral stress shielding, the drill diameter was superior to the stem diameter. Cementation of the stem is only recommended for small size (inferior to 44mm) or after bone grafting of large cysts. On the others cases, we don’t recommended stem cementation to prevent stress shielding. Author recommended a cemented stem for only alignment alone, and not to alignment and force transmission.

Cementation technique recommended was low viscosity cement put directly in the femoral component. A 2.3 minutes period was recommended between cement preparation and femoral impaction. After femoral head impaction, reduction was made without any rotational movement during cement hardening. The leg was left in abduction to improve head to cup impaction. Pulse irrigation during cementation is highly recommended.

Surgical approach consideration

Surgical approach used varied according to each surgeon experience (Fig 3). Hip resurfacing could be be carried out through a posterior, a lateral or an anterior approach although the posterior approach is currently favoured by most surgeons. Author recommended a posterolateral approach because of a facilitated acetabular exposure. No mini invasive surgery was recommended by author. In fact, MIS could led to difficult acetabular exposure and to bad femoral preparation. External rotators were separately divided to the articular capsule. A “T” capsulotomy was made with one superior and one inferior flap. A 2/3 circumferential capsulotomy was then processed. Total circumferential capsulotomy is misadvised because of groin pain risk and anterior retraction of the healed capsule. No exposure of the exogluteal side and no releasing of the tendon of the gluteus maximus muscle from the linea aspera were mandatory. Femoral head was first prepared with a voluntary under preparation of one size. The debulked femoral head is then a lesser obstacle to the acetabular exposure.

Acetabular side was then exposed with an anterior retractor which contains femoral head. Forceful anterior retraction of the femoral head and/or neck should be avoided. Leg was left in external rotation. No under acetabular reaming was recommended because of the bone stock preservation concept. During acetabular preparation, surgeon have to control the reamer position. Abductor muscles and femoral head tend to push the acetabular reamer shaft into excessive abduction and retroversion. This fact could led to wrong cup position with a high risk of metallic ions levels or instability or groin pain. Then, acetabular cup was impacted and finally femoral head was finished with the chamfers. Some drills were made on the femoral sclerotic bone before cementation. No femoral suction was used.

Fig 1: DynaMoM cup design with the double layer porous coating (HA and Titanium plasma spray). There is a cup smooth chamfer on the equatorial area.

Fig 2: Hip resurfacing DynaMoM device. Femoral component could have 3 stems diameter according to femoral size.

Fig 3: Antero-Posterior XRay of a DynaMoM hip resurfacing in a 40 years old woman.