The standard “ISO 7206-13 Implants for surgery - Partial and total hip joint prostheses - Part 13: Determination of resistance to torque of head fixation of stemmed femoral components” developed by the International Organization for Standardization (ISO) describes a method for determining the torque required under specified laboratory conditions to loosen the head fixation of hip joint prostheses in which the head is not intended to rotate relative to the neck. It is applicable to the femoral component of total or partial hip joint replacements in which the head and neck (cone) are fixed together by a locking taper or other means and the head and cone are separate components and made of metallic or non-metallic materials. This standard does not cover methods for examining test specimens, which must be agreed upon between the testing laboratory and the party sending the specimen for testing.
During testing, a static torque is applied to the head/cone assembly of the hip joint prosthesis and the torque is increased until the connection between the head and cone fails or no failure occurs with the selected maximum torque applied.
According to the requirements of this standard, the testing machine must have the following features:
The cone unit consists of a cone of the type to which the header will be mounted during service, or a replica of it with the same dimensions and made of the same material, according to the same manufacturing process and the same specification.
Loading fixtures shall be capable of carrying the expected loads and shall be designed so that the line of action of the external torque is in line with the centre line of the head/cone assembly as shown in the figure given in the standard.
In order to determine the torque required to loosen the fixation of the head of the hip joint prosthesis, at least five test specimens are tested for each cone unit to which the head is to be mounted. Each test specimen shall consist of a head and a neck of the intended fixation of the head of the hip joint prosthesis. Test specimens may consist of actual implants or coupon components.
The assembly of the ball head and cone unit is carried out in accordance with the ISO 7206-10 standard.
The preparation of the head by flattening it in the equatorial region is achieved by obtaining two parallel and plane surfaces having almost the same perpendicular distance from the ball head center. The two surfaces represent a connection for fixing the ball head against axial torque loading with a type of flat key. The perpendicular distance of the parallel surfaces should be in the range of 30-70 percent of the ball head average diameter. The remaining wall thickness on the flat surfaces should be at least 2,0 mm. For a 28 mm ball head, a perpendicular distance of 19,0 mm plus/minus 0,5 mm is considered appropriate.
Alternatively, the head may be prepared by gluing it into a spherical socket of a metal fixture, so that the calotte covers at least 140 degrees of the hemisphere of the head. The failure torque of the joint must exceed the expected failure torque of the head/cone assembly. If the joint interface fails prior to the expected rotation of the head, a new specimen is tested. Epoxy resin adhesives have been found to be suitable for bonding the head to the fastener.
Alternatively, the head can be prepared by embedding it in a embedding fixture using a cast resin. The embedding/head connection failure torque must exceed the expected separation torque of the head/cone assembly. If the connection interface fails prior to the expected rotation of the head, a new specimen is tested. Cast resins have been found to be suitable for fixing the head.
The party submitting the sample for testing defines the acceptance criteria for the test as a torque equal to or greater than the torque of an existing, clinically successful hip head/cone assembly.
Among the numerous testing, measurement, analysis and evaluation studies that our organization has provided for businesses in various sectors, with its trained and expert staff and advanced technological equipment, our organization also provides testing services defined in the standard “ISO 7206-13 Implants for surgery - Partial and total hip joint prostheses - Part 13: Determination of resistance to torque of head fixation of stemmed femoral components”.
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