Objectives

Impingement in total hip replacements (THRs), including bone-on-bone impingement, can lead to complications such as dislocation and loosening. The aim of this study was to investigate how the location of the anterior inferior iliac spine (AIIS) affected the range of motion before impingement.

Methods

A cohort of 25 CT scans (50 hips) were assessed and nine hips were selected with a range of AIIS locations relative to the hip joint centre. The selected CT Scans were converted to solid models (ScanIP) and THR components (DePuy Synthes) were virtually implanted (Solidworks). Flexion angles of 100⁰, 110⁰, and 120⁰ were applied to the femur, each followed by internal rotation to the point of impingement. The lateral, superior and anterior extent of the AIIS from the Centre of Rotation (CoR) of the hip was measured and its effect on the range of motion was recorded.

Materials and Methods

Nine healthy male subjects volunteered for a gait lab-based study to collect kinematics and loading profiles. Owing to limitations in subject selection, five subjects wore a weighted jacket to increase Body Mass Index ≥30 (BMI). An induced increase in Bodyweight was capped (<30%BW) to avoid significantly effecting gait [3] (mean=11%BW).

Six subjects identified as HA per BMI≥30, but with anthropometric ratios indicative of lower body fat as previously detailed by the author [2] (Waist-to-hip circumference ratio and waist circumference-to-height ratio). Three subjects identified as Normal (BMI<25). Instrumented force plate loading profiles were scaled (≈270%BW) in agreement with instrumented hip force data [4].

A previously verified THA (Pinnacle® Marathon® 36×56mm, DePuy Synthes) Finite Element Analysis wear model based on Archard's law and modified time hardening model [5] was used to predict geometrical changes due to wear and deformation, respectively (Figure 1). Subject dependent kinematic and loading conditions were sampled to generate, for both legs, 19 SW simulation runs using a central composite design of response surface method.

Method

A comprehensive anthropometric data set of 4082 men (Gordon, CC., et.al., 2014) was used to characterize a HA population. Obese and HA participants were classed as BMI ≥30 however HA participants were identified by applying anthropometric ratios indicative of lower body fat, namely “waist to height” (i.e. WHtR <0.6) and “waist to hip” (i.e. WHpR <0.9).

Method

Five Zenith (Corin Group PLC), Titanium Nitride (TiN) coated, unconstrained TARs with conventional polyethylene inserts (Figure 1) were tested in an adapted knee simulator (Simulator Solutions, UK) for six million cycles (MC). The input parameters (Figure 2) were taken from available literature as there is no recognised ISO standard in place. A parametric study with three conditions was conducted to understand the impact of kinematic inputs on the polyethylene wear rate. These conditions aimed to understand the effect of both linear wear with isolated flexion, then multidirectional motion by implementing a rotational input with and without anterior/posterior (AP) displacement. Each condition was run for two MC.

Stage One: Flexion and Load

Stage Two: Flexion, Load, Rotation and Displacement

Stage Three: Flexion, Load and Displacement

A lubricant of 25% bovine serum, 0.03% Sodium Azide solution was used to replicate the protein content of the natural joint capsule. The wear was measured gravimetrically every million cycles and surface measurements taken with a contacting profilometer.

Introduction

Leg length and offset discrepancy resulting from Total Hip Replacement (THR) is a major cause of concern for the orthopaedic community. The inability to substitute the proximal portion of the native femur with a device that suitably mimics the pre-operative offset and head height can lead to loss of abductor power, instability, lower back pain and the need for orthodoses. Contemporary devices are manufactured based on predicate studies to cater for the variations within the patient demographic. Stem variants, modular necks and heads are often provided to meet this requirement. The number of components and instruments that manufacturers are prepared to supply however is limited by cost and an unwillingness to introduce unnecessary complexity. This can restrict the ability to achieve the pre-osteoarthritic head centre for all patient morphologies. Corin has developed MiniHip™ to better replicate the physiological load distribution in the femur. This study assessed whether the MiniHip™ prosthesis can better match the pre-osteoarthritic head centre for patient demographics when compared to contemporary long stem devices.

Introduction

This work aimed to mathematically model the force needed to disassemble the THR unit for a given assembly load. This work then compared these results with the results of an in vitro experiment. The research presented aimed to determine the assembly forces necessary to prevent movement of the head on the stem through friction. By assessing the forces necessary to push the head onto the stem securely enough to prevent any movement of the head through friction, it is likely that the fretting and corrosion of the head taper interface will be reduced.

Method:

The Dorr classification is a well accepted clinical method for defining femoral endosteal morphology (5). This is often used by the surgeon to select the appropriate type and size of stem for the individual patient. It is accepted that a strong correlation exists between Flare Index (FI), characterising the thinning of cortical walls and development of ‘stove-pipe’ morphology, and age, in particular for females (Table 1) (3). A statistical model of the proximal femur was built from 30 full length femoral scans (Imorphics, UK). Minimum and maximum intramedullary measurements calculated from the statistical model were applied to relationships produced by combining Corins work with that of prior authors (Table 2) (2; 3; 6). This data was then used to generate 2D CAD models into which implants were inserted to compare the head centres achievable with a MiniHip™ device compared to those of a contemporary long stem.

Methods

Seven metal heads were tested against prototype ϕ52 mm 0.1 wt% vitamin-E blended highly-crosslinked polyethylene liners (Corin, UK). Three heads remained as cast double heat treated metal (MoP) while four, of similar metallurgy, were coated with CrN via electron beam physical vapour deposition (CrNoP) (Tecvac, UK) and polished to a similar surface finish. Tests were conducted for 5 million cycles (mc) under conditions described in ISO 14242–3: 2009. Alumina particles (mean size 2.4 μm) at concentrations of 0.15 mg/mL were added to the lubricant for 1 mc to consider the effect of severe head damage. Testing continued for a further 1 mc without the presence of the particles and then 3 jogging intervals (14,400 cycles each) were conducted at slow, medium and fast speeds [3]. Wear volume was determined gravimetrically for the heads and liners and fluid collected throughout the testing was analysed for cobalt concentration using graphite furnace atomic absorption spectroscopy.

Methods

Suitable head and stem combinations were identified to enable individual variables such as; coating, medial-lateral (M-L) offset, head offset and taper angle to be isolated (Figure 1 and Figure 2). For the coated components a 3 μm CrN coating was applied to trunnion using electron beam physical vapour deposition (Tecvac, Cambridge, UK). Fretting and corrosion testing was carried out in accordance with ASTM F1875-98 (2009) method II procedure B³ following assembly of the components under a 2 kN load.

Method

A dataset consisting of 30 donor scans from a US cadaver tissue bank (ScienceCare, Memphis, US) was used for this study. The dataset consisted of 12 male and 18 female specimens. Due to the limited view of the diaphysis for most scans, the natural slope of the lateral compartment was used as a guide for orienting the resection. All scans were resected with a 3° posterior slope. For the first part of this study, an equal mediolateral (ML) resection of 9.5 mm, reflecting the minimum resection for the Unity TKR tibia (Corin, UK), was performed on all specimens (Figure 1). Following this, two proportionate resection depths (13.5 mm and 6.7 mm) were calculated based on the ML relationship between the smallest and largest available Unity components (59.5 mm: 84.5 mm). Two further resection depths (11.3 mm and 8.0 mm) were calculated based on a mid size (71.0 mm). Three resection depths (8.0 mm, 9.5 mm & 11.3 mm) were applied to four medium sized specimens. In addition to this two larger specimens were resected at 9.5 mm and 13.5 mm and two smaller specimens at 6.7 mm and 9.5 mm.

A grid was applied to all cut surfaces and oriented using the posterior axis. The cut surface was divided based on lines drawn at 10%, 25%, 50%, 75% and 90% of the overall ML dimension and 10%, 25%, 50%, 75% and 90% of the overall anteroposterior (AP) dimension. Measurements were taken from the medial side and recorded from the points at which lines intersected the external profile of the cut tibia (Figure 2).

Methods

Twelve liners (Corin, UK) underwent a 3 million cycle (mc) hip simulation. Three conventional UHMWPE liners (GUR1050, Ø32 mm, 30 kGy sterilised in Nitrogen), three HXLPE liners (GUR1020, Ø40 mm, 75 kGy cross-linking and EtO sterilised) and six ECIMA liners (0.1 wt% vitamin E GUR1020, Ø40 mm, 120 kGy cross-linking, mechanically deformed and annealed, and EtO sterilised) articulated against CoCrMo alloy femoral heads to ASTM F75 (Corin, UK). Wear testing was performed in accordance with ISO 14242 parts 1 and 2, with a maximum force of 3.0 kN and at a frequency of 1 Hz. The test lubricant used was calf serum with a protein content of 30 g/l and 1% (v/v) patricin added as an antibacterial agent. Volumetric wear rate was determined gravimetrically after the first 0.5 mc and every 1 mc thereafter.

ASTM D638 type V specimens (3.2 mm thick) were machined from ECIMA material for uniaxial tension testing to ASTM D638. Ultimate tensile strength (UTS), yield strength and elongation values were measured. These values were compared to mechanical data available for the other material types.

METHODS

Twelve liners (Corin, UK) underwent a 3 million cycle (mc) hip simulation. Three UHMWPE (GUR1050, Ø32 mm, γ sterilised), three HXLPE (GUR1020, Ø40 mm, 75 kGy γ, EtO sterilised) and six ECIMA (0.1 wt% vitamin E GUR1020, Ø40 mm, 120 kGy γ, mechanically annealed, EtO sterilised) liners articulated against CoCrMo femoral heads (Corin, UK). Wear testing was performed in accordance with ISO 14242 parts 1 and 2, in calf serum, with a maximum force of 3.0 kN and at a frequency of 1 Hz. Volumetric wear rate was determined gravimetrically.

ASTM D638 type V specimens were machined from ECIMA material for uniaxial tension testing. Ultimate tensile strength (UTS), yield strength and elongation values were measured. These values were compared to mechanical data available for the other material types.

Following completion of the ECIMA wear testing, three of the tested liners were cut in half. One half of each was subject to accelerated ageing in accordance with ASTM F2003-02, while the other half was tested as received. Each liner half was cross-sectioned and a microtome was used to section 200μm thick slices from each cross-section. Oxidation analysis was performed using a Fourier Transform Infra-red technique in accordance with ASTM F2102-01 throughout the thickness of each liner half. Average oxidation indices for each sample were determined.