Patellar Luxation Grade 4: Palpation Grades and X-Ray…

Grade 4 patellar luxation is the one grade you cannot reduce by hand. The kneecap sits permanently outside a trochlear groove that is often shallow or flat, and pressure alone will not put it back. That single finding changes the X-ray series, the age at which surgery is planned, and how much femur has to be reshaped. A 30-second palpation done the same way at every visit is what separates grade 3 from grade 4.

Four grades and what each one means

Grading describes where the patella sits and what it does when you let go. It says nothing about pain, cartilage wear, or how well the cat walks. Record the grade, the side, and the examiner, because a grade that changes between visits usually means the technique changed, not the knee. Most orthopedic services use the same 4-step scale.

Grade	Patella position	Reduces by hand	Usual age at first sign (months)
1	In the groove; pops out only when pushed	Yes, and it stays	8-18
2	Out when standing; back in when pushed	Yes, then pops out again	5-12
3	Out most of the time; returns with leverage	Yes, but relapses in seconds	3-9
4	Out and fixed; groove empty under the skin	No, at any angle	2-7

Grades 1 and 2 are what owners notice as an occasional skip. Grade 3 is a cat that carries the leg for 2 or 3 steps and then walks normally. Grade 4 is a cat that has never put the foot down flat, or lost the ability to do so between 3 and 7 months of age.

Palpation in 30 seconds

Stand behind the cat with the stifle extended and the hock at about 90 degrees. Put the thumb on the medial side of the patella and the index finger laterally, then push the patella toward the midline with steady pressure for 2 to 3 seconds. In grade 2 the patella slides 5 to 8 mm and returns on its own. In grade 3 it travels further, needs a small rotation of the tibia to come back, and slips out again within 1 or 2 seconds. In grade 4 nothing moves: the groove feels like a flat shelf, the patella is fixed 10 to 15 mm lateral to where it belongs, and pushing harder only flexes the stifle.

Repeat the test 3 times, once standing and twice in lateral recumbency. A patella that reduces at 30 degrees of flexion but not at 90 is a grade 3 with a tight quadriceps, not a grade 4, and that difference decides whether a soft tissue procedure alone is enough.

Why grade 4 is a different disease

In grade 4 the whole extensor mechanism is displaced. The quadriceps pulls 20 to 30 degrees off the femoral axis, the tibial tuberosity sits 5 to 10 mm medial to the groove, and the groove is often under 2 mm deep instead of the normal 3 to 5 mm. The medial side of the joint takes the load, so cartilage loss starts within 2 to 4 months and many cats are lame before their first birthday. Most surgeons treat this as a deformity to correct rather than a kneecap to pop back: deepen the groove, move the tibial tuberosity 5 to 10 mm lateral, and add a femoral osteotomy when the angle is over 25 degrees.

X-ray: 3 views, each in its own position

The mediolateral view is taken with the stifle flexed 45 to 60 degrees, beam centered on the joint, cassette against the limb; that angle lifts the patella clear of the condyles and lets you measure patellar height. The craniocaudal view needs the stifle at 85 to 95 degrees with the beam angled 10 degrees cranial, which opens the groove and shows medial displacement. The skyline or tangential view, at 70 to 90 degrees of flexion with the beam parallel to the groove, is the only one that shows depth and the height of the ridges. Shoot each view twice if the cat shifts more than 2 to 3 mm between exposures, and keep the 6 films for the 6 to 8 week recheck.

Numbers to read off the film

Four measurements carry most of the decision. The anatomic lateral distal femoral angle is 95 to 98 degrees in a normal cat; over 102 degrees suggests varus that will pull the patella medially again. Tibial tuberosity offset, measured from the center of the groove, is normally under 3 mm, while grade 4 cats often show 6 to 12 mm. A groove under 2 mm deep on the skyline view predicts that soft tissue repair alone will fail. Joint space narrowing of 1 mm or more with 2 or more osteophytes means arthritis is already established, and the owner should hear that before a date is booked.

Timing at 4, 8 and 12 weeks

Most grade 4 cats are operated between 6 and 10 months, once the growth plates are closing and the femur can hold an implant. After surgery the usual plan is 2 weeks of cage rest with 3 to 5 short leash walks a day, 6 weeks of controlled walking, and a recheck film at 6 to 8 weeks to confirm the patella is still centered. Passive flexion starts at week 2, 10 repetitions twice a day. A second film at 12 weeks is common after a femoral osteotomy, because that is when the bone is solid enough to judge the correction.

When to call the vet today

A cat that suddenly carries the leg completely, cries when the stifle is touched, or has swelling that grows over 12 to 24 hours needs to be seen the same day; fracture, infection, and a locked luxation all look like this. A kitten under 4 months with a fixed patella should be seen within 1 to 2 weeks, not at the next vaccine visit. If the lame cat also stops passing urine, that is a separate emergency and the signs are in urinary blockage signs. A head tilt or nystagmus along with the lameness points elsewhere, and vestibular disease covers the 3 patterns to check.

Five steps for this week

Ask the vet to grade both stifles and write the grade, side, and date in the record.
Film 10 seconds of the cat walking from behind and from the side, so week 4 and week 8 can be compared.
Weigh the cat and hold the weight steady; every extra 500 g adds load to a joint that is already off axis.
Book the 3-view series before deciding on surgery, and take all 6 films to the orthopedic consult.
Read the cost estimate before the consult, so the plan is chosen on need rather than on budget shock.

Terms mentioned in this article

Frequently asked

Can a grade 4 patella ever be pushed back into place?

No. By definition the patella is fixed outside the groove and manual pressure at any stifle angle fails to move it back. If it does reduce with leverage, the cat is grade 3, and that changes the surgical plan from a reshaping procedure to a soft tissue repair.

Which X-ray views are needed before surgery?

Three: a mediolateral view at 45 to 60 degrees of flexion, a craniocaudal view at 85 to 95 degrees with the beam angled 10 degrees cranial, and a skyline view at 70 to 90 degrees. Together they show patellar height, medial displacement, and groove depth, and each view should be repeated if the cat moves.

How soon should a kitten with a fixed kneecap be seen?

Within 1 to 2 weeks, not at the next scheduled visit. A patella that never sits in the groove during the first 4 months usually comes with a shallow groove and a rotated tibial tuberosity, and early assessment keeps more surgical options open.

Patellar Luxation Grade 4: Palpation Grades and X-Ray Views