Fracture Between a Hip and a Knee Implant
- An interprosthetic femoral fracture is a fracture of the femoral shaft occurring BETWEEN an ipsilateral HIP implant (a total hip or hemiarthroplasty stem) and a KNEE implant (a total knee femoral component); it is rare but devastating, with incidence rising as more patients have both a hip and a knee replacement and live longer.
- The fundamental biomechanical concept is that the bone TRAPPED between two implants is a STRESS RISER: a SHORT inter-implant (inter-stem) distance concentrates stress, and osteoporosis, osteolysis, cortical defects (including old screw holes), loose implants and a fall all increase the risk - so prevention includes avoiding leaving a short, unsupported inter-implant segment.
- Assessment must determine the FRACTURE LOCATION and, critically, the FIXATION STATUS of BOTH implants (is the hip stem well-fixed or loose? is the knee femoral component well-fixed?), because implant stability - more than the fracture pattern alone - drives the treatment decision; the hip side is often classified with Vancouver-type thinking (B1 well-fixed, B2/B3 loose stem) and the distal/knee side by remaining bone stock and component fixation.
- When BOTH implants are WELL-FIXED, treatment is OPEN REDUCTION AND INTERNAL FIXATION, and the key principle is to use a SPANNING construct - a long locked plate that bridges essentially the WHOLE femur (overlapping or engaging both implants) - so that the fixation does NOT create a new short, unsupported segment (a new stress riser) between the plate end and an implant.
- When an implant is LOOSE or the bone is deficient, REVISION/REPLACEMENT is required: a loose hip stem is treated with revision to a long stem (that may bypass the fracture), and a loose knee component or deficient distal femoral bone is treated with a distal femoral replacement (megaprosthesis), with total femoral replacement reserved for the worst combined bone loss.
- Outcomes are POOR with HIGH REOPERATION rates: a large series reported only around 71% two-year survivorship free of reoperation, with periprosthetic joint INFECTION the commonest reason for reoperation (also nonunion, refracture, hardware failure), and fractures occurring between TWO STEMMED components carried the highest reoperation and infection risk - so these are high-risk reconstructions requiring careful planning, robust fixation and infection vigilance.
- “Interprosthetic femoral fracture = femoral shaft fracture BETWEEN an ipsilateral hip and knee implant; the inter-implant bone is a STRESS RISER (short inter-stem distance, osteoporosis).
- “Assess BOTH implants' fixation - implant stability drives treatment. Well-fixed both -> ORIF with a SPANNING plate bridging the WHOLE femur (don't create a new stress riser); loose hip stem -> revision long stem; deficient distal bone -> distal/total femoral replacement.
- “High reoperation rate (~71% 2-yr survivorship free of reop); PJI the commonest cause; fractures between two STEMMED components are highest risk.
Femoral shaft fracture between a hip and a knee implant - the inter-implant bone is a stress riser (worse with a short inter-stem distance and osteoporosis).
Assess both implants' fixation. Well-fixed -> ORIF with a spanning plate bridging the whole femur (no new stress riser); loose/deficient -> revision / distal-femoral replacement.
The Stress Riser, Assessment & Treatment Principle
An interprosthetic femoral fracture occurs in the femoral shaft between an ipsilateral hip implant and a knee implant, and the bone trapped between them is a stress riser - a short inter-implant distance, osteoporosis and cortical defects all increase the risk. Assessment must define the fracture location and the fixation status of BOTH implants, because implant stability drives treatment (hip side: Vancouver-type B1 well-fixed vs B2/B3 loose; distal side: bone stock and knee-component fixation). With both implants well-fixed, treat by ORIF using a spanning locked plate that bridges essentially the whole femur so the construct does NOT create a new short unsupported segment. With a loose implant or deficient bone, revise/replace: a loose hip stem -> long revision stem; deficient distal bone/loose knee -> distal femoral replacement. These are high-risk reconstructions with high reoperation and infection rates.
Operative Strategy
- Assess both implants' fixation (and bone stock) on imaging - this, not the fracture pattern alone, determines treatment.
- Both implants well-fixed -> ORIF with a spanning construct: a long locked plate bridging essentially the whole femur, overlapping/engaging both implants, so no new short unsupported segment (stress riser) is created; use locking screws around stems and cables where needed.
- Loose hip stem -> revision to a long stem (bypassing the fracture by adequate cortical diameters).
- Loose knee component or deficient distal bone -> distal femoral replacement (megaprosthesis); total femoral replacement for the worst combined bone loss.
- Anticipate complications: high reoperation rate, with periprosthetic joint infection the commonest cause (also nonunion, refracture, hardware failure) - meticulous technique, biology and infection prophylaxis."
The defining technical error in fixing an interprosthetic femoral fracture is to use a construct that leaves a new short, unsupported segment of bone between the end of the plate and an implant - this simply creates a fresh stress riser and predisposes to a refracture. The principle is therefore to SPAN the whole femur, bridging or engaging both implants with a long locked plate (and cables/locking screws around the stems), rather than using a short plate confined to the fracture. Equally, the decision between fixation and revision/replacement hinges on implant stability and bone stock, so both implants must be assessed: a loose hip stem needs revision and a deficient distal femur needs a distal femoral replacement, not ORIF. Finally, because reoperation - most often for infection - is common, these reconstructions demand careful planning, robust biology-respecting fixation and infection vigilance.
Evidence & Key Studies
High rate of unplanned reoperation for interprosthetic femur fractures
- In 76 surgically managed interprosthetic femur fractures, treatment followed implant stability: well-fixed (Vancouver B1/C) fractures had ORIF, loose stems (B2/B3) had revision, and deficient distal bone was treated with distal femoral replacement.
- The 2-year survivorship free of reoperation was only 71%, with periprosthetic joint infection the commonest reason for reoperation (also refracture, nonunion, hardware failure, instability).
- Fractures involving a stemmed femoral knee component (i.e. between two stemmed implants) had a significantly increased risk of reoperation and infection; ORIF and revision had similar survivorship.
Stress concentration and inter-implant distance in (inter)prosthetic femoral fractures
- With both a hip stem and a knee prosthesis in the femur, the short inter-implant (inter-stem) distance concentrates stress and predisposes to interprosthetic/periprosthetic fracture.
- Prophylactic plate reinforcement can still fail if it does not adequately protect the stress-concentration region (e.g. a plate ending at the subtrochanteric region with a hip stem in situ).
- Awareness of stress-concentration points is important for both prevention and the design of fixation constructs.
According to PubMed, the implant-stability-driven treatment algorithm (ORIF for well-fixed, revision for loose stems, distal femoral replacement for deficient distal bone), the high reoperation rate (about 71% two-year survivorship free of reoperation) with periprosthetic joint infection the commonest cause, and the highest risk between two stemmed components come from the cited Neitzke series; the stress-concentration/inter-implant- distance concept and the caution that inadequately positioned reinforcement can still fail from the cited Taniguchi report. The spanning-fixation principle (bridge the whole femur to avoid a new stress riser) and the Vancouver-type assessment of the hip side are standard, well-established teaching. (See also our Periprosthetic Hip Fracture (Vancouver) and Periprosthetic Knee Fracture topics.)
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
“What is an interprosthetic femoral fracture and what determines its treatment?”
“What are the technical principles and the expected complications of fixing one?”
Mnemonics & Memory Aids
SPAN
Hook:SPAN: Stress riser, Probe both implants, All the femur (span it), Not fixable if loose -> revise/replace (watch infection).
Concept
- Femoral shaft fracture between an ipsilateral hip and knee implant
- Inter-implant bone is a stress riser (short inter-stem distance, osteoporosis, cortical defects)
- Rising incidence (ageing, multiply-replaced patients)
Assessment
- Fracture location + fixation status of BOTH implants (and bone stock)
- Hip side: Vancouver-type (B1 well-fixed, B2/B3 loose)
- Implant stability drives treatment (not fracture pattern alone)
Treatment
- Both well-fixed -> ORIF with a spanning plate bridging the WHOLE femur (no new stress riser)
- Loose hip stem -> revision long stem (bypass fracture)
- Loose knee/deficient distal bone -> distal (or total) femoral replacement
Outcomes
- High reoperation rate (~71% 2-yr survivorship free of reoperation)
- Periprosthetic joint infection the commonest cause; also nonunion/refracture/hardware failure
- Highest risk between two stemmed components