Standardized Loads Acting in Hip Implants

Abstract

With the increasing success of hip joint replacements, the average age of patients has decreased, patients have become more active and their expectations of the implant durability have risen. Thus, pre-clinical endurance tests on hip implants require defining realistic in vivo loads from younger and more active patients. These loads require simplifications to be applicable for simulator tests and numerical analyses. Here, the contact forces in the joint were measured with instrumented hip implants in ten subjects during nine of the most physically demanding and frequent activities of daily living. Typical levels and directions of average and high joint loads were extracted from the intra- and inter-individually widely varying individual data. These data can also be used to analyse bone remodelling at the implant-bone interface, evaluate tissue straining in finite element studies or validate analytical loading predictions, among other uses. The current ISO standards for endurance tests of implant stems and necks are based on historic analytical data from the 1970s. Comparisons of these test forces with in vivo loads unveiled that their unidirectional orientations deviate from the time-dependent in vivo directions during walking and most other activities. The ISO force for testing the stem is substantially too low while the ISO force for the neck better matches typical in vivo magnitudes. Because the magnitudes and orientations of peak forces substantially vary among the activities, load scenarios that reflect a collection of time-dependent high forces should be applied rather than using unidirectional forces. Based on data from ten patients, proposals for the most demanding activities, the time courses of the contact forces and the required cycle numbers for testing are given here. Friction moments in the joint were measured in addition to the contact forces. The moment data were also standardized and can be applied to wear tests of the implant. It was shown that friction only very slightly influences the stresses in the implant neck and shaft.

Fig 1. Coordinate system of right femur and implant.

x, y, z = axes of femur coordinate system. x = parallel to posterior contour of condyles. P1 = intersection of neck axis and femoral midline. P2 = middle of intercondylar notch. z = straight femur axis between P1 and P2. Force components F_x, F_y and F_z act in directions x, y and z. Moment components M_x, M_y and M_z turn clockwise around x, y and z. The implant is turned clockwise by angles α_z, α_y and α_x around the femur axes z, y and x. α_z = anteversion of neck (negative). x’, y’, z’ = axes of implant. x_n, y_n, z_n = coordinate system at distal end of implant neck. x_s, y_s, z_s = coordinate system of stem 80 mm below head centre.

Fig 2. Average force AVER75 and calculation of high loads HIGH100.

Resultant forces F_res from walking. Coloured lines = forces from 10 subjects with normalized BW = 75 kg. Black solid line = AVER75 force, represents subject of 75 kg with average load levels. Subject H7R had the highest peak value (red circle). AVER75 was multiplied by factor f₁, to obtain a peak equal to the peak of H7R (black dotted line). The obtained force was further multiplied by f₂ = 1.33 = 100 kg / 75 kg to obtain the HIGH100 load (black dashed line). The factor f_AH = f₁*f₂, obtained from the analysis of F_res, was then applied to all 6 load components. HIGH100 represents subjects of 100 kg with high load levels. For explanation of factors f₁ and f₂, please also see Fig 3.

Fig 3. Peak forces F_res from nine activities.

Coloured lines = individual loads from 10 subjects with normalized BW = 75 kg. Black solid line = average loads AVER75 for BW = 75 kg. Black dashed line = high loads HIGH100 for BW = 100 kg. Factors f₁ and f₂ are shown for the data from standing up; for other activities, they are different. See also Fig 2.

Fig 4. High100 forces during nine activities and ISO forces.

High forces for a simulated subject with a body weight of 100 kg. Components F_x, F_y and F_z and resultant force F_res in the femur coordinate system.

Fig 5. HIGH100 loads during one gait cycle of walking.

Top = 3 force components and resultant force. Bottom = 3 moment components and resultant moment. To obtain average loads AVER75 instead of the high loads HIGH100, all components must be divided by the factor f_AH (Table 5).

Fig 6. High100 moments during nine activities.

High loads for a simulated subject with a body weight of 100 kg. Components F_x, F_y and F_z and resultant force F_res.

Fig 7. Peak force and path of force vector during one cycle of walking.

HIGH100 forces from the average subject. Left = frontal plane. Middle = sagittal plane. Right = transverse plane. The vector acts from the green trajectory towards the head centre. ‘Start’ and circle = point on trajectory at the instant of heel strike. Red dots = directions of the peak force. Dashed lines = limits of force directions throughout the cycle for forces > 1500 N. Angles α_front, α_sagit, α_trans = angles of the force vector in 3 planes.

Fig 8. Magnitude and direction of HIGH100 peak forces during 9 activities.

High loads HIGH100 from the average subject. Left = frontal plane. Middle = sagittal plane. Right = transverse plane. Vectors of peak forces act from the symbols (different activities) towards the head centre. Red vector = direction during walking. Small red dots = AVER75 peak forces from individual subjects during walking, indicating the inter-individual variation of force directions for the same activity.