Aims

Periprosthetic fracture and implant loosening are two of the major reasons for revision surgery of cementless implants. Optimal implant fixation with minimal bone damage is challenging in this procedure. This pilot study investigates whether vibratory implant insertion is gentler compared to consecutive single blows for acetabular component implantation in a surrogate polyurethane (PU) model.

Taper corrosion and Trunionnosis are recognized as a major complication of hip replacement surgery presenting in a variety of clinical manifestations commonly referred to as Adverse Local Tissue Reactions. Metal debris is produced through Mechanically Assisted Crevice Corrosion with several implicating factors including mixed alloy components, taper design, head offset, femoral head size, and taper impaction techniques (including magnitude of force, control of alignment and environmental factors). Our project has focused singularly on taper impaction techniques and surgeon controlled factors, as we believe the process of head impaction unto a trunnion is non-standardized, which often times dooms the trunionn to failure. We have contemplated a standardization process, such that given the right tool, the surgeon can control the quality of the taper interlock, which may produce a “cold weld” or perfect taper interlock, eliminate micro motion, mechanically assisted crevice corrosion, and trunionnosis. We have considered four specific problems with current head to trunionn impaction techniques: 1. The magnitude of applied force is uncontrolled, haphazard, and non-standardized. 2. Non-axial application of force is the norm, which produces canting, leading to micro-motion and tribocorrosion. 3. The transfer of energy from the head to the trunionn interface is highly inefficient, such that the energy produced by the surgeon is mostly dissipated in a non-constrained system. 4. No in vitro studies exist to guide surgeons as to the magnitude of force required for a proper interlock.

Regardless of the design, including taper angles, larger heads, offset heads, mixed alloy components, shorter and slimmer trunionns there is a widespread problem with the process of head impaction onto the trunionn and the engagement of the modular taper interface that dooms the trunionn interface to failure. The deficiencies noted in current techniques are addressed with a simple tool and minor modification of the femoral stem. We present a new concept/apparatus for head to trunionn taper assembly that fully controls the magnitude and direction of assembly force within a constrained, dry and contaminant free environment. This tool allows application of a perfectly axial and high insertional forces without risk of damage to the femoral stem/bone interface to obtain a cold weld and perfect taper interlock with no chance for canting, micro motion and tribocorrosion. The concept has been verified through several prototypes and can be adopted in order to standardize the process of taper assembly, making this procedure independent of surgeon skill and strength, and minimizing the incidence of trunionnosis.

Purpose

Current methods for inserting a press fit hemispherical metal-backed acetabular component within the acetabula are uncontrolled, relying on the surgeon to generate the necessary forces required for sufficient introduction. While previous studies have recorded impact forces of 2–3 kN necessary to seat an acetabular cup using visual observation[1], some researchers have observed users imparting as high as 8.9 kN of force[2]. The aim of this study is to quantify the forces required to generate optimal implant primary stability, as well as compare force delivery methods.

Cup implantation is a critical stage during THR Surgery. It is mainly because of the rudimentary mallet-based impaction technique, whereby mal-positioning and unknown forces are present. There are some technological attempts to solve this problem partially: dealing with mal-positioning while patient remains subjected to non-standardized impaction forces.

Our comprehensive approach to the problem allows the surgeon to monitor cup positioning and perform controlled insertion with a completely known force profile. Positioning is monitored by means of IMU (Inertial Measurement Unit) technology, while placement is controlled by force feedback and vibratory insertion device. Both technological building blocks (IMU and vibratory insertor) are embedded on a single device containing signal processing and automatic control strategies. This mechatronic device is called BMD3.

This work covers the entire device development life cycle illustrated in figure 1: the roadmap starts at the conceptual inspiration through scientific investigation and concept proof/demonstration up to the BMD3 prototype.

Smooth insertion was the main purpose initially; this led to concept demonstration by means of electrical and pneumatic actuated devices. They employ low-amplitude/high-frequency vibratory input forces into the Acetabular Cup to explore constant sliding in the microscale. Although successful, it was noted that there is optimization potential as vibration is used to decrease friction resistance and either impose or prevent specific shape modes on the pelvic structure. A scientific investigation on frictional and structural behavior allowed us to define suitable instrumentation for an automatic insertion strategy (figure 2a).

Our technical solution to the smooth insertion problem involves positioning monitoring by means of IMU, simultaneously available to the surgeon while using this tool. An operating procedure was proposed to reliably map and feedback surgeon's movement in the OR (Operating Room) space. Concept demonstration was also performed for this additional feature before complete device integration, see figure 2b.

Three main subsystems compose the BMD3: PPU (Power and Processing Unit); Mechatronic Handle; and Replaceable Head. The Replaceable Head allows 1kHz and 20kHz operating ranges; each implemented on a specific mechanism detachable from the Mechatronic Handle. A user (surgeon) may choose one of these versions according to the insertion strategy adopted.

The Mechatronic Handle houses sensors, initial signal conditioning stages, and surgeon interaction interfaces like: Thin Film Transistor screen for visual positioning feedback; and a pushbutton for OR space mapping setup. The Mechatronic Handle itself is an interface between Replaceable Head and the PPU. Every insertion and positioning strategy may be updated directly at the PPU; firmware updates deal with real-time processing of pressure, IMU, and vibration measurements.

Conclusions of the work summarize intangibles such as inspiration and insights on THR improvement spots; scientific analysis; and technology to the effective problem approach.

Total hip replacement (THR) is one of the most successful orthopedic operations, yet it continues to be plagued with problems despite the many advances in the procedure. Inconsistent placement of the acetabular cup persists even in the hands of most experienced surgeons, leading to early and late failure including instability, impingement, polyethylene wear, osteolysis, and component loosening. Cup mal-position is the single greatest cause of early instability and late polyethylene wear. Despite advent of recent technology including navigation and use of fluoroscopy cup mal-alignment persists. Several studies show 50% of experienced surgeons missing the target ranges using Lewinnnk desired safe zones. The act of impaction of the cup with a mallet is a crude and unreliable process. The surgeon's mallet imparts large and uncontrolled forces on the impaction rod creating variable torques, leading to inconsistent cup placement. Navigation and Fluoroscopy add precision to the operation however that level of precision is not maintained throughout the course of the operation. There is a market need for a tool that helps maintain “precision tolerance” through out the course of the operation.

A new device is theoretically proposed and prototyped for this process (Patent Pending). The new paradigm involves elimination of impaction forces created by unpredictable blows of the mallet. A low energy and high frequency device is utilized to insert and position the acetabular cup without the use of the mallet. The cup is inserted (not impacted) with significantly less force than the typical 2000N forces created with a mallet. The cup is also simultaneously positioned to the desired alignment while the device is active with the surgeon effectively feeling minimal haptic resistance to the movement of the cup. The system therefore proposes to eliminate cup mal-alignment for all surgeons, removing the primary cause of hip dislocations as well as factors contributing to late failure. In addition the idea allows the academic surgeon to better study the relationship of the position of the cup and clinical outcomes eliminating the need to use “safe zone ranges”. As well, this process completely eliminates acetabular fractures as a complication of this operation.

Two devices were prototyped with use of electrical and pneumatic energy. Both devices proved the concept. Both devices allowed modulation of the applied force and “effective” disarming of the frictional forces involved in cup impaction, allowing insertion and positioning of the acetbular cup to occur with smooth haptic control and without the use of violent force. The device can be used individually, with navigation and fluoroscopy, with robots and/or with any other intra-operative measurement device and can be a significant adjunct for THR.

Cup Mal-Alignment is an unsolved problem in THR surgery causing poor outcomes for patients, anxiety and a sense of failure for the surgeons, and a great cost to society in general. A new device is described to solve this problem. The science involved is proposed and described in detail and primarily involves understanding and utilizing the mechanical properties of bone/pelvis and understanding and manipulating the complex frictional forces at play.