The Kyon Zurich cementless total hip replacement (THR) system has constantly evolved since its release around 25 years ago. Over this time there have been modifications to the stem design to reduce the risk of stem failure, HA (hydroxyapatite) coating of the implants has been adopted and the bearing surface has gone from titanium on polyethylene to the current ceramic on carbon fibre reinforced polyether ether ketone (CFR PEEK). The current 6th generation implants were introduced in 2019. The changes over the past 5-6 years include:
The Bearing Surface
The 5th generation cup introduced CFR PEEK as the cup liner in 2016 and the major advance of the 6th generation of the system was the adoption of VeriloxÒ ceramic (zirconia toughened alumina) heads. Ceramic on PEEK offers reduced liner wear compared to CoCr on cross-linked polyethylene1,2. PEEK however is stiffer/less compliant and has greater surface friction than polyethylene2,3. These features required a change in acetabular reaming technique when PEEK cups were introduced. If the cup is impacted into a ‘tight’ acetabular ream there can be a noticeable increase in friction between the head and the cup in some cases. The 5th generation cups therefore required a redesign of the acetabular reamers which are marginally larger than the previous generation reamers such that impaction does not deform the cup and liner. More recently (2023) the geometry of the liner at its margins has been altered to avoid ‘pinch points’ after impaction.
PEEK wear particles have similar bioreactivity to polyethylene wear particles4–6 but the considerably lower volumetric wear associated with PEEK should reduce the risk of bone-implant interface degradation (aseptic loosening) and therefore improve implant longevity.
The standard Kyon system includes two head sizes. Traditionally the smaller cups (21.5, 23.5 and 26.5) accepted a 16mm diameter head and the larger cups accepted a 19 mm head. The switch to a PEEK liner has allowed the 26.5 cup to be redesigned such that from 2019 the 26.5 cup accepts a 19 mm head, thus reducing the risk of luxation in medium sized dogs.
Dual Mobility Cup
In man there is a greater risk of luxation after THR in hip dysplasia patients and after revision THR surgery. Dual mobility cups reduce the luxation risk in these patients.7–9
The Kyon dual mobility cup is available in 29.5 and 32.5 sizes and is therefore suitable for large and giant breed dogs. The cup has an inner polished titanium surface which accepts a 25 mm CFR PEEK head which comes preassembled with an internal 19 mm ceramic head. The PEEK head captures the 19 mm ceramic head such that the ceramic head cannot readily luxate from the PEEK head. The dual mobility head is mounted on a standard neck. The dual mobility head has a greater range of motion and is less likely to impinge on the cup than a single head, due to the altered head-to-neck ratio. This and the greater jump distance (the translation of the head required to result in luxation) improves stability.
The results of the dual mobility cup have been reported in 105 cases.10 Median body weight was 43 kg (range 30-64 kg). Follow up was available for 103 hips (median 5 months, range 2-27 months). There was one dislocation in a Bernese Mountain Dog 27 months after surgery and no other complications. Long term longevity of the outer PEEK head is currently unknown but its replacement is feasible if necessary. Good implant longevity has been reported in human dual mobility THR.11
Mini Cementless THR
The Kyon mini THR system introduced 3, 4, 5, and 6 mm stems and 10, 12, 14, 16, 18 and 20 mm cups matched to heads ranging from 6 – 16 mm. There is a high degree of modularity between the different stem/cup/head sizes.
The mini stems are essentially scaled-down versions of the standard stems. The 18 mm and 20 mm mini cups are very similar to the larger cups (double shell titanium/CFR PEEK) whereas the smaller cups are one-piece PEEK with a Ti/HA coating. The small size of mini cups is a concern from a wear perspective and I have personally seen cemented micro THRs with polyethylene liners fail due to cup wear. This should be much less likely with a one-piece PEEK cup.
Compared to cemented micro/nano THR and standard cementless THR there is much less room for error with the cementless Kyon mini system. The small size of the implants means that precise reaming and careful technique are essential and the pelvis needs to be stabilised well to place the press-fit cup, which can be more difficult in smaller patients. A mini version of the Biomedtrix positioning board is now available which may help with this.
The Kyon mini THR system and initial results in 59 hips have been described.12 There were three intraoperative trochanteric fractures, repaired with a plate each time. Median follow-up time for dogs was 8 months (range 2-17 months) and for cats (11 hips) was 10 months (4-16 months). Postoperative complications were seen in 6 dogs (12.5%) and 1 cat (9%). The cat had a femoral fracture distal to the stem 5 days after surgery, which was repaired. The complications in dogs included one cup dislocation (20 mm cup), two ventral luxations, one failure of cup integration (21 mm cup), one stem peg failure (3 mm stem), one stem peg wearing which led to separation of the neck from the stem (3 mm stem, peg design on this stem has since been modified). The 3mm stem was used in 9 dogs overall (and 2 cats) and may have a higher complication rate than the other sizes. The overall complication rate was 12%.
Large Breed Dogs
To bridge the gap between the standard 8 mm stems and the 11 mm ‘Tiger’ stem (for dogs upto 90 kg), in the last few years Kyon introduced ‘Giant’ stems. These include an XL 8 mm stem (for dogs upto 55 kg) and an XXL 9 mm stem (upto 60 kg). Both have an enlarged stem peg and therefore require giant-specific necks to match to a 19 mm standard or dual mobility head.
Compatibility
Kyon and Biomedtrix are now both owned by Movora and both companies have increased options for cross-compatibility between their systems. 17 mm and 22 mm Kyon heads and 16 mm and 19 mm Biomedtrix heads are available.