Pilliar et al. used two independent canine models to define how much movement at the implant-bone interface determines fixation type. The central question: how much micromotion is too much for bone ingrowth to occur?
The radiopaque halo you see around a cementless stem is not an incidental finding — it is the radiographic signature of fibrous rather than bony fixation, the direct consequence of exceeding these micromotion thresholds early after implantation.
When you see this pattern on a postoperative hip film, think about why: was the stem undersized? Did the patient bear weight before six weeks? Was bone stock poor? Those are the modifiable risk factors Pilliar's data identified.
This paper is the mechanobiological foundation for why we pursue a tight press-fit in cementless arthroplasty and why early weight-bearing restrictions are not arbitrary. The permissive window for bone ingrowth is narrow. Under 28 μm. And anything above 150 μm commits the interface to fibrous fixation.
The most dangerous clinical scenario is not complete fibrous fixation, but mixed fixation: fibrous proximally and bone ingrowth distally in a stemmed implant. That pattern raises strain at the transition zone and is the mechanistic explanation for late implant fatigue fracture.
Pilliar et al. used two independent canine models to define how much movement at the implant-bone interface determines fixation type. The central question: how much micromotion is too much for bone ingrowth to occur?
The radiopaque halo you see around a cementless stem is not an incidental finding — it is the radiographic signature of fibrous rather than bony fixation, the direct consequence of exceeding these micromotion thresholds early after implantation.
When you see this pattern on a postoperative hip film, think about why: was the stem undersized? Did the patient bear weight before six weeks? Was bone stock poor? Those are the modifiable risk factors Pilliar's data identified.
This paper is the mechanobiological foundation for why we pursue a tight press-fit in cementless arthroplasty and why early weight-bearing restrictions are not arbitrary. The permissive window for bone ingrowth is narrow. Under 28 μm. And anything above 150 μm commits the interface to fibrous fixation.
The most dangerous clinical scenario is not complete fibrous fixation, but mixed fixation: fibrous proximally and bone ingrowth distally in a stemmed implant. That pattern raises strain at the transition zone and is the mechanistic explanation for late implant fatigue fracture.